Glossary

Accretion

The growth of an astronomical object as surrounding matter falls toward it under gravity; around black holes, the infalling material can become extremely hot and luminous.

Accretion is the gravitational accumulation of gas, dust, or other material onto an astronomical object. Around a black hole, orbiting matter can form a disk or other inflow in which friction, magnetic fields, and turbulence convert gravitational energy into heat and radiation before the material crosses the event horizon. This process powers active galactic nuclei and quasars.

Related: Active galactic nucleus

Active galactic nucleus

An unusually luminous compact region at the center of a galaxy, powered by matter falling toward a supermassive black hole.

An active galactic nucleus, or AGN, is the energetic central region of a galaxy in which gas and dust accreting onto a supermassive black hole produce intense radiation. Quasars are among the most luminous forms of AGN. The black hole itself does not emit the observed light; most of the radiation comes from surrounding material heated as gravitational energy is converted into heat and motion.

Related: Accretion, Redshift

Adipose tissue

Body tissue specialized for storing fat that also produces hormones and other signaling molecules.

Adipose tissue contains fat-storing adipocytes along with blood vessels, immune cells, connective tissue, and other cell types. It stores and releases energy, but also acts as an endocrine and immune organ by sending signals to the brain, liver, muscle, and other tissues.

Aether

In Aristotelian cosmology, the incorruptible fifth element assigned to the heavens, distinct from earthly earth, water, air, and fire.

Aristotle's aether was the material of the celestial realm. Unlike the four terrestrial elements, which were associated with change and straight-line natural motions, aether was supposed to be ungenerated, incorruptible, and naturally capable of eternal circular motion. This made it possible to treat stars and planets as physically different from earthly matter.

Related: Quintessence

Afterglow

Fading radiation observed after the initial flash of a transient event such as a gamma-ray burst.

A gamma-ray burst afterglow is longer-lived emission that follows the prompt gamma-ray flash and can be observed at X-ray, optical, radio, and other wavelengths. Because it persists much longer than the burst itself, astronomers can localize the event, identify its host galaxy, and obtain spectra for distance measurements.

Related: Redshift

Alchemy

A diverse premodern tradition combining practical work with matter, medicine, metallurgy, and theories of transformation, including attempts to transmute metals.

Alchemy was not a single doctrine but a family of traditions practiced across different cultures and periods. It combined hands-on techniques such as distillation, heating, alloying, and purification with theories about the composition and transformation of matter. In European and Islamic traditions, one prominent goal was metallic transmutation, although alchemical practice also included medicines, pigments, salts, and other materials.

Related: Transmutation

Analgesia

Reduction or prevention of pain without necessarily causing loss of consciousness.

Analgesia means relieving or preventing pain. It can be produced by medicines or other interventions and does not by itself imply unconsciousness; a patient can be awake while receiving effective analgesia.

Related: Anesthesia

Ancient DNA

DNA recovered from archaeological, fossil, or historical remains rather than from living organisms.

Ancient DNA, often abbreviated aDNA, is genetic material extracted from old biological remains such as bones, teeth, preserved tissues, or sediments. Because DNA degrades and contamination is a major risk, ancient-DNA research uses specialized laboratory and statistical methods. Genome-scale datasets can reconstruct ancestry, population turnover, migration, domestication, and evolutionary relationships through time.

Related: Yamnaya

Anesthesia

The medical use of drugs or techniques to prevent pain during procedures; some forms also produce unconsciousness.

Anesthesia is a broad term for medical techniques that block sensation during procedures. General anesthesia produces unconsciousness, while regional or local anesthesia can block pain in part of the body without making the patient unconscious.

Related: Analgesia

Antiperistasis

A historical Aristotelian-style explanation in which surrounding air displaced by a projectile circulates behind it and helps keep it moving.

Antiperistasis was one way medieval commentators described the Aristotelian attempt to explain projectile motion after contact with the thrower ended. The displaced medium was supposed to move around the projectile and continue pushing it from behind. The model became a target for later critics because the resisting medium was also being asked to serve as the continuing mover.

Related: Impetus

Astrometry

The precise measurement of the positions and motions of objects in the sky.

Astrometry measures where celestial objects are and how those positions change over time. It underlies measurements of stellar proper motion and parallax and is central to missions such as ESA's Gaia.

Bacillus

A rod-shaped bacterium; historically the word was often used descriptively in names such as 'tubercle bacillus.'

In its descriptive microbiological sense, bacillus means a rod-shaped bacterium. The word also appears in historical disease terminology, including 'tubercle bacillus' for Mycobacterium tuberculosis. It should not be confused with Bacillus, the formal genus name.

Bipedalism

Locomotion on two legs; in human evolution, habitual bipedalism means regularly using upright two-legged walking as a normal form of movement.

Bipedalism is movement using two legs. Human habitual bipedalism is reflected in multiple anatomical adaptations involving the pelvis, spine, femur, knee, foot, and skull. These traits evolved gradually, and evidence for regular upright walking predates the large increase in brain size in later Homo.

Related: Hominin

Bone histology

The microscopic study of bone tissue, including growth marks and tissue structure that can provide evidence about an animal's age and growth rate.

Bone histology examines thin sections of fossil or living bone under a microscope. Features such as vascularization, growth lines, remodeling, and changes in tissue organization can help researchers reconstruct growth history and assess whether an individual was still growing or approaching skeletal maturity. In paleontology, these signals are useful but must be interpreted alongside anatomy and specimen preservation.

Related: Holotype

Carbon nanotube

A nanoscale cylindrical structure made from a sheet-like network of carbon atoms.

Carbon nanotubes are tube-shaped carbon allotropes with walls built from a graphene-like hexagonal carbon network. Their geometry gives them mechanical, electrical, and thermal properties very different from bulk graphite or diamond.

Related: Fullerene

Cardiomyocyte

A specialized muscle cell of the heart that generates force for the heartbeat.

Cardiomyocytes are the contractile muscle cells that make up much of the heart's working muscle. Adult human cardiomyocytes can be replaced, but only slowly, which is why normal turnover does not provide enough regeneration to rebuild a large area destroyed by a major heart attack.

Catalyst

A substance that increases the rate of a chemical reaction without being consumed overall by the reaction.

A catalyst provides a reaction pathway with a lower activation barrier, allowing a reaction to proceed faster under given conditions. The catalyst participates in intermediate steps but is regenerated overall. In biology, most catalysts are protein enzymes, although some RNA molecules are catalytic as well.

Related: Ribozyme

Cepheid variable

A pulsating star whose brightness changes in a regular cycle related to its intrinsic luminosity, making it useful for measuring astronomical distances.

Cepheid variables brighten and dim with a period closely related to their intrinsic luminosity. Comparing that luminosity with how bright the star appears lets astronomers estimate its distance. Cepheids became an important rung in the cosmic distance ladder used to measure distances beyond direct geometric methods.

Chemosynthesis

The production of organic matter using energy from chemical reactions rather than energy from sunlight.

Chemosynthesis allows some microorganisms to build organic molecules using energy released by chemical reactions, for example by oxidizing hydrogen sulfide, methane, or other reduced compounds. These organisms can form the base of food webs in environments where sunlight is absent, including some deep-sea ecosystems.

Related: Hydrothermal vent

Chymistry

The early-modern study and manipulation of matter before chemistry and alchemy became sharply separated disciplines.

Historians use chymistry as an intentionally archaic umbrella term for the early-modern field that included activities later divided among chemistry, alchemy, metallurgy, pharmacy, and theories of matter. The spelling helps avoid imposing modern disciplinary boundaries on practitioners who did not recognize them in the same way.

Climate feedback

A process triggered by climate change that then amplifies or reduces the original temperature response.

A climate feedback is a response within the climate system that changes the size of an initial warming or cooling. Positive feedbacks amplify the original change, while negative feedbacks oppose it. Important examples for climate sensitivity include water-vapor, lapse-rate, cloud, and surface-albedo feedbacks.

Related: Equilibrium climate sensitivity

Climax community

In classical succession theory, the mature, relatively stable community expected to form at the endpoint of ecological succession.

The climax community was central to Frederic Clements's influential model of succession. Vegetation was expected to develop through ordered stages toward a mature community determined chiefly by regional climate. Ecologists still use climax terminology in some contexts, but the idea of one inevitable permanent endpoint has been superseded by models incorporating disturbance, multiple trajectories, and alternative states.

Related: Ecological succession

Clovis

An early archaeological culture of North America known especially for distinctive fluted stone projectile points dating to roughly 13,000 years ago.

Clovis refers to an archaeological complex found across much of North America and characterized especially by fluted stone points. During much of the twentieth century, Clovis sites were widely treated as evidence for the continent's first human population. Securely dated older sites later showed that people were present in the Americas before Clovis.

Related: Pre-Clovis

Coelacanth

A lineage of lobe-finned fishes known from fossils and represented today by two living species in the genus Latimeria.

Coelacanths are lobe-finned fishes with a fossil record extending hundreds of millions of years. They were long known only from fossils and were thought to have disappeared tens of millions of years ago until a living specimen was recognized in 1938. Modern coelacanths are not unchanged fossil species; they are living members of an ancient lineage that has continued evolving.

Cortical map

An organized representation in the cerebral cortex in which different regions correspond to different sensations, body areas, movements, or other information.

A cortical map describes the orderly way information is represented across the cerebral cortex. Sensory and motor maps, for example, associate neighboring cortical regions with particular parts of the body or sensory input. These maps are not perfectly fixed and can reorganize when inputs, behavior, or injury change.

Related: Neuroplasticity

Cosmic microwave background

Faint microwave radiation left from the early universe, released when the cosmos became transparent about 380,000 years after the Big Bang.

The cosmic microwave background, or CMB, is relic radiation from the hot early universe. It was released when matter cooled enough for electrons and nuclei to combine into neutral atoms, allowing photons to travel freely. Tiny temperature and polarization variations in the CMB preserve information about the early universe and strongly constrain the amounts of ordinary matter, dark matter, dark energy, and other cosmological parameters.

Related: Dark matter, Dark energy

Cryovolcanism

Volcanic-like activity on cold worlds in which water, water vapor, ice particles, or other volatile materials erupt instead of molten silicate rock.

Cryovolcanism is a broad term for eruption-like geological activity involving volatile materials on icy worlds. Instead of hot silicate lava, the erupted material may include water, vapor, ice grains, ammonia, methane, or mixtures of these. Enceladus's south-polar jets are a prominent example of active volatile release from an icy moon, though researchers distinguish among specific eruption mechanisms.

Related: Tidal heating

Dark energy

The name for whatever is causing the observed accelerated expansion of the universe in the standard cosmological model.

Dark energy is the label for the component of the standard cosmological model associated with the universe's accelerated expansion. In the simplest model it behaves like a cosmological constant with nearly uniform energy density throughout space. Its physical origin is unknown, and the term does not imply that it is a form of dark matter.

Related: Dark matter, Cosmic microwave background

Dark matter

Matter inferred from its gravitational effects that does not emit, absorb, or reflect enough light to be seen directly.

Dark matter is the name for matter whose presence is inferred mainly from gravity rather than emitted light. Evidence includes galaxy rotation, motions in galaxy clusters, gravitational lensing, the cosmic microwave background, and large-scale structure. Its microscopic nature remains unknown, and it is distinct from dark energy.

Related: Gravitational lensing, Cosmic microwave background, Dark energy

Deep-inelastic scattering

High-energy scattering in which a probe penetrates a proton or neutron deeply enough to reveal point-like constituents inside it.

In deep-inelastic scattering, high-energy electrons, muons, neutrinos, or other probes transfer enough energy and momentum to a nucleon that its internal structure becomes visible in the scattering pattern. Experiments beginning in the late 1960s found behavior consistent with compact constituents inside protons, helping establish the quark picture.

Related: Nucleon, Quantum chromodynamics

Depleted mantle source

A mantle reservoir that has lost some elements during earlier melting and crust formation, leaving it relatively poor in those easily extracted components.

In geochemistry, a depleted mantle source is a mantle reservoir from which previous melting removed part of the elements that preferentially enter melts. Its later magmas therefore carry characteristic elemental and isotopic signatures. Chang'e-5 basalts were traced to a depleted lunar mantle source with little evidence for a KREEP component.

Related: KREEP, Procellarum KREEP Terrane

Diffraction limit

The conventional resolution limit caused by the wave nature of light, which makes point-like objects appear as finite blurred spots.

In ordinary optical imaging, diffraction spreads light from a point object into a finite pattern. Two sufficiently close objects therefore produce overlapping patterns that cannot be cleanly distinguished by conventional imaging. Ernst Abbe related this resolution scale to wavelength and numerical aperture. Super-resolution methods do not make diffraction vanish; they obtain additional information by changing illumination, fluorescence behavior, or reconstruction.

Related: Super-resolution microscopy, STED, PALM, STORM

Double bind

A historical communication theory proposing that repeated contradictory messages in close relationships could contribute causally to schizophrenia.

The double-bind hypothesis proposed that a person, especially a child, could be trapped by recurring incompatible messages in which responding to one level of communication violated another and the contradiction could not safely be addressed. The idea became influential in family theories of schizophrenia but did not survive as a supported causal explanation of the disorder.

Related: Schizophrenogenic mother

Druids

Religious and learned specialists described by Greek and Roman writers among Iron Age societies in Gaul and Britain.

The historically documented Druids belong to Iron Age societies known through classical written sources. Stonehenge’s principal stone settings are thousands of years older, so this historical group cannot be identified as the monument’s builders.

Dwarf planet

Under the IAU's Solar System definition, a body that orbits the Sun, is nearly round from its own gravity, but has not cleared its orbital neighbourhood.

The International Astronomical Union introduced the formal dwarf-planet category in 2006. A dwarf planet orbits the Sun and has enough self-gravity to become nearly round, but unlike a planet it has not cleared other comparable bodies from the neighbourhood of its orbit. Pluto, Eris, Haumea, Makemake, and Ceres are recognized examples.

Related: Kuiper Belt

Ecological succession

Change in the species composition and structure of an ecological community over time, often after disturbance or the creation of new habitat.

Ecological succession describes directional change in communities through time as species arrive, disappear, compete, modify their environment, and respond to disturbance. Older theories often pictured succession as a predictable march toward a single climax community. Modern ecology allows for multiple trajectories, recurring disturbance, historical contingency, and alternative long-lasting states.

Related: Climax community

Ejecta

Material thrown outward from an impact or eruption; impact ejecta can include shattered, melted, or vaporized rock.

In impact geology, ejecta is material expelled from a crater-forming collision. Its distribution, chemistry, and shock effects can preserve evidence of an impact far from the crater itself.

Electron diffraction

A technique that infers atomic-scale structure from the pattern produced when a beam of electrons scatters from a material.

Electrons have wave-like behavior, so a beam passing through or reflecting from an ordered material can interfere to form a diffraction pattern. The positions and symmetries of the resulting spots or rings reveal information about atomic arrangement and crystal structure. Electron diffraction was central to the discovery of quasicrystals because their sharp pattern showed strong order together with symmetry incompatible with ordinary periodic crystals.

Element

A word whose scientific meaning changed: classical 'elements' were basic kinds or principles of matter, whereas a modern chemical element is defined by the number of protons in its atoms.

In ancient and medieval natural philosophy, an element could mean one of a small set of fundamental material principles such as earth, water, air, and fire. Modern chemistry uses the same word much more narrowly: a chemical element is a substance whose atoms share the same atomic number, meaning the same number of protons in the nucleus. Air is therefore not a modern element but a mixture of several gases, including elemental nitrogen and oxygen.

Related: Aether

Episiotomy

A surgical incision made in the perineum to enlarge the vaginal opening during childbirth.

An episiotomy is a surgical cut through the perineal tissues during the second stage of labor to enlarge the vaginal opening. It was once performed routinely in many settings in the belief that a controlled incision would prevent more serious tearing and protect the pelvic floor. Evidence favored restrictive rather than routine use for spontaneous vaginal birth.

Related: Perineum

Equilibrium climate sensitivity

The long-term global warming expected after atmospheric carbon dioxide is doubled and the climate system has largely adjusted.

Equilibrium climate sensitivity, or ECS, is the change in global mean surface temperature after a sustained doubling of atmospheric CO₂ once the climate approaches equilibrium. It includes fast climate feedbacks such as changes in water vapor, clouds, sea ice, and lapse rate, while very slow responses such as major ice-sheet changes are usually treated separately.

Related: Climate feedback

Eukaryote

An organism whose cells keep their DNA inside a nucleus; animals, plants, fungi, and many single-celled organisms are eukaryotes.

Eukaryotes are organisms made of cells that contain a nucleus and other internal membrane-bound compartments. Animals, plants, fungi, and many single-celled lineages are eukaryotes. Bacteria and archaea belong to different cellular lineages and do not have a nucleus.

Related: Molecular phylogeny

Event horizon

A boundary around a black hole beyond which light and other signals cannot escape to distant observers.

An event horizon is a causal boundary in spacetime. Crossing it does not require hitting a material surface, but after crossing it no future-directed signal can reach the distant exterior universe.

Related: Schwarzschild radius, General relativity

Exon

A section of a gene's initial RNA copy that is retained when introns are removed during RNA splicing.

In a split gene, exons are the sections retained in the mature RNA after intervening introns are removed. Exons can be joined in different combinations through alternative splicing, so one gene can sometimes produce multiple RNA products.

Related: Intron, RNA splicing

Fomite

An inanimate object or surface that can carry infectious material and contribute to transmission.

A fomite is an inanimate object or surface that can become contaminated and help transmit an infectious agent. How important fomite transmission is depends strongly on the pathogen; for yellow fever, contaminated objects were historically suspected but mosquitoes proved to be the important route.

Related: Vector

Fossil flora

Fossil evidence of past plant life, including leaves, wood, pollen, spores, roots, seeds, and other preserved plant remains.

Fossil flora is the preserved record of plants from past environments. Different plant remains provide different kinds of information: leaves and wood can reveal vegetation type, pollen and spores can document plant communities, and roots or fossil soils can show that plants grew locally rather than being transported there. Together these remains are powerful evidence for reconstructing ancient climates and ecosystems.

Related: Paleoclimate

Free radical

An atom, molecule, or molecular fragment with one or more unpaired electrons.

A free radical contains at least one unpaired electron, which often makes it chemically reactive. Radical lifetimes vary enormously: many are extremely short-lived, while others can persist when their unpaired electron is delocalized or when bulky molecular groups shield the reactive region. Stable and persistent radicals are important in organic chemistry, materials science, biology, and atmospheric chemistry.

Fullerene

A carbon molecule whose atoms form a closed hollow cage, such as the roughly spherical C60 molecule.

Fullerenes are molecular allotropes made entirely of carbon atoms arranged in closed cages. The 1985 discovery of C60 showed that carbon could form stable structures beyond the long-known diamond and graphite forms.

Related: Carbon nanotube

General relativity

Einstein's theory of gravity, in which matter and energy curve spacetime and that curvature governs how objects and light move.

General relativity is Einstein's theory of gravitation. Instead of treating gravity simply as a force acting across space, it describes mass, energy, and pressure as shaping spacetime, with objects and light following paths through that curved geometry. Black-hole solutions arise from the theory under sufficiently extreme gravitational conditions.

Related: Schwarzschild radius, Event horizon

Graphene

A two-dimensional material consisting of a single atomic layer of carbon arranged in a hexagonal lattice.

Graphene is a one-atom-thick sheet of carbon atoms bonded in a honeycomb-like hexagonal lattice. It is the structural building block underlying graphite and can exist as a free-standing or supported two-dimensional crystal. Its unusual electronic, mechanical, and thermal properties helped launch the modern field of atomically thin materials.

Related: Fullerene, Carbon nanotube

Gravitational lensing

The bending and distortion of light by gravity as it passes near mass.

General relativity predicts that mass curves spacetime and therefore bends the paths of light rays. In gravitational lensing, foreground mass changes the apparent position, shape, or brightness of more distant sources. Measuring those distortions lets astronomers map total gravitating mass, including matter that emits little or no light.

Related: Dark matter

Gray

The SI unit of absorbed ionizing-radiation dose. One gray (Gy) means one joule of radiation energy absorbed per kilogram of matter; one nanogray (nGy) is one-billionth of a gray.

Gray measures how much ionizing-radiation energy is deposited in matter. It is an absorbed-dose unit, not by itself a measure of biological harm; sieverts (Sv) are used for radiation quantities that apply biological weighting. In the background-radiation card, 214 nGy per hour means an absorbed dose rate of 214 billionths of a joule per kilogram each hour. Popular "banana equivalent dose" comparisons are normally framed in sieverts, so they are not a direct conversion from gray.

Helicobacter pylori

A spiral-shaped bacterium that can colonize the stomach lining and is a major cause of chronic gastritis and peptic ulcers.

Helicobacter pylori is a bacterium adapted to living in the human stomach. Persistent infection can cause chronic inflammation of the stomach lining and is one of the two most common causes of peptic-ulcer disease. Treating the infection with appropriate antibiotics and acid-suppressing therapy greatly reduces ulcer recurrence.

Related: NSAID

Holotype

The single specimen designated as the name-bearing reference when a new species or subspecies is formally described.

In zoological nomenclature, a holotype is the single specimen fixed as the name-bearing type of a new nominal species or subspecies when it is established. It anchors the application of the scientific name; it is not necessarily the most complete, typical, or best-preserved specimen known.

Related: Bone histology

Hominin

A member of the human evolutionary lineage after its split from the lineage leading to chimpanzees and bonobos.

Hominin is the standard term for humans and extinct species more closely related to humans than to chimpanzees and bonobos. The group includes the genus Homo as well as earlier lineages such as Australopithecus and other candidates near the human–chimpanzee divergence.

Related: Bipedalism

Hydrothermal vent

An opening in the seafloor where geothermally heated, mineral-rich water flows into the ocean.

Hydrothermal vents form where seawater circulates through hot oceanic crust, is heated and chemically altered, then returns through openings in the seafloor. Some vent ecosystems are supported by chemosynthetic microorganisms rather than sunlight.

Igneous rock

Rock formed when molten rock—magma or lava—cools and solidifies.

Igneous rocks form by the cooling and solidification of molten rock. Magma that cools underground can form intrusive rocks such as granite, while lava that cools at or near the surface forms volcanic rocks such as basalt. Crystal size and texture depend strongly on composition and cooling history.

Related: Sedimentary rock, Metamorphic rock, Neptunism

Impetus

A medieval theory in which a mover imparts a continuing motive tendency to a projectile, allowing it to keep moving after contact ends.

In Jean Buridan's influential fourteenth-century impetus theory, the thrower impresses an impetus into the projectile. That impressed quality keeps the object moving until resistance and other influences diminish or overcome it. Impetus was an important step away from external air-push explanations, but it was not yet Newtonian inertia because continued motion was still attributed to a positive internal cause.

Related: Antiperistasis

Incorruptible

In Aristotelian natural philosophy, incapable of decay, generation, or fundamental physical alteration.

Incorruptible did not merely mean durable. In Aristotelian cosmology it described the celestial realm as exempt from the generation and decay characteristic of earthly matter. The heavens were therefore supposed to remain fundamentally unchanged, a view challenged by observations of novae, comets, sunspots, and lunar topography.

Related: Sublunary, Aether

Intron

An intervening section copied into a gene’s initial RNA transcript that is usually removed during RNA splicing before the mature RNA is produced.

Introns interrupt the retained parts of many eukaryotic genes. They are transcribed into precursor RNA but are normally removed before the mature messenger RNA is used for protein production.

Related: Exon, RNA splicing

Ionizing radiation

Radiation energetic enough to remove electrons from atoms or molecules, creating ions and potentially damaging biological tissue.

Ionizing radiation includes X-rays, gamma rays, and sufficiently energetic particles. By ionizing atoms and molecules directly or through secondary particles, it can alter chemical bonds and damage DNA. Biological risk depends on radiation type, dose, dose rate, exposed tissue, age, and other factors.

Isomer

One of two or more compounds that share the same molecular formula but differ in how their atoms are connected or arranged in space.

Isomers contain the same kinds and numbers of atoms but differ in molecular structure. Constitutional isomers differ in which atoms are bonded to which, while stereoisomers have the same connectivity but different three-dimensional arrangements. Because structure affects interactions and energy, isomers can have very different physical, chemical, and biological properties despite sharing a formula.

Jebel Irhoud

A fossil site in Morocco that yielded early Homo sapiens remains dated to roughly 315,000 years ago.

Jebel Irhoud in Morocco preserves human fossils and Middle Stone Age artifacts dated to about 315,000 years ago. The fossils combine relatively modern facial and dental features with a more archaic braincase. Their age and anatomy helped extend the known roots of Homo sapiens toward 300,000 years and broaden the geography of early human evolution beyond East Africa.

Related: Mosaic anatomy

KREEP

A geochemical component of lunar material enriched in potassium (K), rare-earth elements (REE), phosphorus (P), and several heat-producing radioactive elements.

KREEP is the conventional name for lunar material enriched in potassium, rare-earth elements, and phosphorus. It is also associated with incompatible elements including uranium and thorium, whose radioactive decay produces heat. KREEP therefore matters to models of the Moon's chemical differentiation and thermal history, although Chang'e-5 showed that the youngest sampled mare basalts did not require a KREEP-rich mantle source.

Related: Procellarum KREEP Terrane, Depleted mantle source

Kuiper Belt

A broad region beyond Neptune containing many icy small bodies, including Pluto and other dwarf planets.

The Kuiper Belt is a population of icy bodies orbiting the Sun beyond Neptune, mostly between roughly 30 and 50 astronomical units. Discoveries beginning in the 1990s showed that Pluto is part of a much larger family of trans-Neptunian objects rather than an isolated oddity, helping trigger the modern debate over the definition of a planet.

Related: Dwarf planet

Last Glacial Maximum

The most recent interval when global ice sheets reached their greatest extent, roughly 26,500 to 19,000 years ago.

The Last Glacial Maximum, or LGM, was the coldest major phase near the end of the last ice age, when large continental ice sheets covered much of northern North America and Eurasia and global sea level was far lower than today. Evidence for humans in North America during this interval has major implications for migration routes and the chronology of the peopling of the Americas.

Related: Luminescence dating

Lidar

A remote-sensing method that measures distance with laser pulses to build detailed three-dimensional maps.

Lidar sends laser pulses toward a surface and measures the returning light to calculate distance. Airborne lidar can record enough returns through gaps in vegetation to reconstruct ground topography and reveal archaeological features hidden beneath forest canopy.

Luminescence dating

A dating method that estimates when mineral grains such as quartz were last exposed to light or intense heat.

Luminescence dating measures trapped electronic charge that accumulates in minerals through natural background radiation. Exposure to sunlight or sufficient heat resets much of that stored signal. By measuring the remaining luminescence and the environmental radiation dose, researchers can estimate the time since sediment was last exposed to light. At White Sands, optically stimulated luminescence provided an independent check on the footprint chronology.

Related: Last Glacial Maximum

Mariner 2

A NASA spacecraft that flew past Venus in 1962 and returned the first successful close-range measurements of another planet.

Mariner 2 became the first successful interplanetary spacecraft when it flew past Venus on 14 December 1962. Its microwave and infrared measurements supported the conclusion that Venus has an extremely hot surface, sharply weakening earlier expectations of a mild Earth-like world beneath the clouds.

Related: Venera

Metagenomics

The study of genetic material taken directly from a mixed sample, without first growing each microorganism separately in the laboratory.

Metagenomics reads DNA from entire communities in samples such as seawater, soil, or the human gut. It can reveal organisms and genes that routine laboratory culture misses.

Metamorphic rock

Rock transformed from an older rock by heat, pressure, fluids, or a combination of these without fully melting.

Metamorphic rocks begin as pre-existing igneous, sedimentary, or older metamorphic rocks. Elevated temperature, pressure, stress, and reactive fluids can recrystallize minerals and change the rock's texture or composition. If the rock melts completely, the resulting melt belongs to the igneous cycle instead.

Related: Igneous rock, Sedimentary rock

Microcephaly

A condition in which the head and usually the brain are substantially smaller than expected for age and sex because brain growth was reduced or disrupted.

Microcephaly is a clinical finding in which head size is markedly below the expected range, usually reflecting reduced brain growth. It can result from many genetic, developmental, infectious, or environmental causes. In the debate over Homo floresiensis, microcephaly was proposed as one possible explanation for the very small brain of the LB1 skeleton.

Molecular phylogeny

The reconstruction of evolutionary relationships using molecular data such as DNA, RNA, or protein sequences.

Molecular phylogeny compares inherited molecular sequences to estimate how organisms are related and where their evolutionary lineages diverged.

Related: Eukaryote

Monotreme

An egg-laying mammal of the group that includes platypuses and echidnas.

Monotremes are mammals that reproduce by laying eggs rather than giving live birth. Living monotremes are the platypus and echidnas. They possess defining mammalian traits such as hair and milk production while retaining an ancestral reproductive mode that differs from marsupials and placental mammals.

Mosaic anatomy

A combination of ancestral and more derived anatomical traits in the same organism or fossil.

Mosaic anatomy describes organisms that combine traits at different evolutionary stages rather than fitting a simple all-old or all-modern pattern. In early Homo sapiens fossils, for example, relatively modern facial features can occur alongside a more archaic braincase. Such combinations are evidence that evolutionary change often proceeds in different anatomical systems at different rates.

Related: Jebel Irhoud

Mycologist

A scientist who studies fungi.

Mycology is the branch of biology concerned with fungi; a researcher working in that field is a mycologist.

Neptunism

A late-18th-century geological theory that treated most major rocks as deposits precipitated from a vast primordial ocean.

Neptunism is associated especially with Abraham Gottlob Werner. In its influential form, Earth's major rock units were arranged as a sequence deposited as a universal ocean receded. The theory explained some layered rocks well but struggled with evidence that granite, basalt, and other rocks had formed through heat, intrusion, or volcanism.

Related: Igneous rock, Sedimentary rock, Metamorphic rock

Neuroplasticity

The nervous system's capacity to change its structure, connections, or function in response to experience, learning, injury, or other conditions.

Neuroplasticity is the capacity of the nervous system to reorganize. It can involve changes in synaptic strength, connectivity, cortical representations, and measurable aspects of brain structure. Plasticity continues in adulthood, although its extent depends on age, brain region, experience, injury, disease, and training.

Related: Cortical map

Neutrino flavor

One of the three interaction types of neutrino: electron, muon, or tau.

Neutrinos are observed in three flavors—electron, muon, and tau—named for the charged leptons with which they participate in weak interactions. A flavor state is not the same thing as a mass state: each flavor is a quantum mixture of mass states. That mismatch makes neutrino oscillation possible.

Related: Neutrino oscillation

Neutrino oscillation

The quantum-mechanical change of a neutrino from one flavor to another as it travels.

Neutrino oscillation occurs because neutrino flavor states are mixtures of states with different masses. Those mass states evolve with different quantum phases during travel, so the probability of detecting a particular flavor changes with distance and energy. Observing oscillations therefore proves that the relevant neutrino mass states cannot all have identical mass.

Related: Neutrino flavor, Super-Kamiokande

Non-REM sleep

The sleep stages outside REM, ranging from light sleep to deep slow-wave sleep.

Non-REM sleep includes several stages with progressively different brain activity, muscle tone, and ease of awakening. Deep non-REM sleep is associated with slow-wave activity. Dream experiences can occur during non-REM sleep as well as REM, although their reported qualities and frequency often differ.

Related: REM sleep

NSAID

A common class of pain-relieving and anti-inflammatory medicines that includes drugs such as aspirin, ibuprofen, and naproxen.

Nonsteroidal anti-inflammatory drugs, or NSAIDs, reduce pain, fever, and inflammation by inhibiting cyclooxygenase enzymes and lowering prostaglandin production. The same mechanism can weaken protective processes in the stomach and duodenum, so repeated or high-risk use can contribute to ulcers and gastrointestinal bleeding.

Related: Helicobacter pylori

Nucleon

A collective name for the two particles that make up ordinary atomic nuclei: protons and neutrons.

Protons and neutrons are collectively called nucleons because both occupy the atomic nucleus and participate in the strong nuclear interaction. They are not fundamental particles: each is a composite hadron whose internal structure is described in terms of quarks and gluons.

Related: Deep-inelastic scattering, Quantum chromodynamics

Oscillation mode

A characteristic pattern and frequency at which a physical system can naturally vibrate.

A planet can support many normal or oscillation modes, each with its own spatial pattern and natural frequency. Their frequencies depend on the planet's internal structure. As a giant planet evolves, slowly changing mode frequencies can interact resonantly with orbiting moons and influence tidal migration.

Related: Resonance locking

Paleoclimate

The climate of Earth in the geological past, reconstructed from physical, chemical, and biological evidence.

Paleoclimate research reconstructs climates that existed before modern instrumental records. Evidence can come from ice cores, sediments, fossils, pollen, tree rings, soils, isotopes, and other proxies. Such records show that Earth's climate has varied greatly through time and allow scientists to test how climate responds to greenhouse gases, continental geography, ocean circulation, and orbital changes.

Related: Fossil flora

PALM

A super-resolution method that localizes sparse subsets of photoactivatable fluorescent molecules and combines many cycles into a fine image.

Photoactivated localization microscopy, or PALM, activates only a sparse subset of fluorescent labels at a time so that their diffraction-blurred spots do not strongly overlap. Each molecule's position can then be estimated with high precision. Repeating the cycle and combining thousands of localizations reconstructs a super-resolved image.

Related: Super-resolution microscopy, STORM, Diffraction limit

Parallax

The apparent shift in an object's position against a more distant background when viewed from different locations.

Parallax is a geometric effect caused by changing the observer's viewpoint. Nearby objects appear to shift more against distant background objects than faraway ones do. Astronomers can therefore use parallax to estimate distance. Tycho Brahe's failure to detect the large parallax expected for a nearby atmospheric comet helped place the Great Comet of 1577 beyond the Moon.

Parity

A symmetry principle asking whether the laws governing a process remain the same when every spatial direction is reflected as in a mirror.

In particle physics, parity transforms a physical system into its spatial mirror image. If an interaction conserves parity, the mirrored process is governed in the same way as the original. Electromagnetic and strong interactions conserve parity to very high precision, but the weak interaction can violate it, meaning nature can distinguish left from right in weak processes.

Related: Weak interaction

Perineum

The region of tissue at the bottom of the pelvis; in childbirth discussions, the term commonly refers to the tissue between the vaginal opening and anus.

Anatomically, the perineum is the diamond-shaped region below the pelvic floor between the thighs. In obstetric use, 'perineum' often refers more narrowly to the tissue between the vaginal opening and anus that stretches during vaginal birth and may tear or be incised during an episiotomy.

Related: Episiotomy

Phlogiston

A hypothetical fire-like substance once thought to be contained in combustible materials and released during burning.

Phlogiston theory gave seventeenth- and eighteenth-century chemists a common framework for combustion, calcination, smelting, and respiration. Combustible substances were supposed to contain phlogiston and lose it when burned. Quantitative mass measurements and the oxygen theory of combustion eventually made the hypothetical substance unnecessary.

Planck constant

A fundamental constant of quantum physics that sets the proportionality between a photon's energy and its frequency.

The Planck constant, symbol h, connects energy and frequency through the relation E = hν. Since the 2019 SI redefinition, its numerical value is fixed exactly, allowing the kilogram to be realized through precision measurements rather than defined by a physical artifact.

Plasma

A gas in which enough atoms or molecules are electrically charged that the material behaves collectively as charged particles.

Plasma forms when a gas contains substantial numbers of free electrons and ions. Because those charged particles respond strongly to electric and magnetic fields, plasma can behave differently from an ordinary neutral gas. Much diffuse material in space is plasma.

Polychromy

The use of multiple colors in a work of art or architecture.

In classical sculpture, polychromy can include painted pigments, gilding, inlays, and selective coloring of details such as clothing, hair, eyes, or skin.

Pre-Clovis

A broad archaeological label for human presence in the Americas that predates the Clovis culture.

Pre-Clovis is not the name of a single people or culture. It is a chronological label for archaeological evidence older than Clovis. Sites such as Monte Verde helped establish that humans were present in the Americas before the Clovis horizon, while the exact timing and routes of the earliest migrations remain debated.

Related: Clovis

Prion

An infectious protein form that can propagate by inducing normally folded versions of the same protein to adopt the same abnormal shape.

A prion is an infectious agent composed principally of protein and lacking its own DNA or RNA genome. Its propagation depends on protein conformation: an abnormal form can induce normally folded copies of the host protein to adopt the same misfolded state. The host genome still encodes the normal protein; what the infectious agent lacks is a separate nucleic-acid genome.

Procellarum KREEP Terrane

A large region on the Moon's near side with unusually high concentrations of KREEP-associated elements, including thorium.

The Procellarum KREEP Terrane is a geochemically distinctive region covering much of the Moon's western near side. Remote sensing found high abundances of thorium and other KREEP-associated elements there, and many young mare basalts occur within the region. That geographic association helped motivate the idea that KREEP-related heat was important for prolonged lunar volcanism.

Related: KREEP, Depleted mantle source

Quantum chromodynamics

The quantum theory of the strong interaction, describing how quarks interact through gluons.

Quantum chromodynamics, usually abbreviated QCD, is the part of the Standard Model that describes the strong interaction. Quarks carry a property called color charge and interact by exchanging gluons. QCD explains why quarks are confined inside hadrons such as protons and neutrons and why much of a nucleon's mass arises from strong-interaction energy rather than from the bare masses of its quarks.

Related: Nucleon, Deep-inelastic scattering

Quintessence

Literally the 'fifth essence'; a later term associated with the heavenly fifth element beyond earth, water, air, and fire.

Quintessence derives from the Latin quinta essentia, 'fifth essence.' In medieval and early-modern natural philosophy it was used for a fifth substance beyond the four terrestrial elements and was often associated with the incorruptible celestial aether. The term also developed other meanings in alchemy and medicine, so it is related to but not always exactly interchangeable with Aristotle's original aether.

Related: Aether

Radiation dose

A quantity used to describe how much ionizing radiation energy is deposited in matter or, in protection contexts, the biological significance of an exposure.

Radiation dose is a family of measurement concepts rather than a single number. Absorbed dose measures energy deposited per unit mass and is expressed in gray. Radiation protection also uses equivalent and effective dose, expressed in sievert, to account for radiation type and the differing sensitivities of tissues. Repeated exposures contribute to a person's cumulative radiation history even though biological risk is not determined by dose alone.

Related: Ionizing radiation

Radiocarbon dating

A method for dating once-living material by measuring how much radioactive carbon-14 remains in it.

Living organisms continually exchange carbon with the environment. After death that exchange stops, and carbon-14 decays at a known rate, allowing organic remains to be dated over archaeological timescales.

Redshift

A shift of observed light toward longer wavelengths; for distant galaxies, cosmological redshift mainly reflects the expansion of space during the light's journey.

Redshift describes an increase in the observed wavelength of light. It can arise in several ways, including relative motion, gravity, and cosmic expansion. For distant galaxies and gamma-ray burst hosts, cosmological redshift provides a key measure related to distance and the expansion history of the universe.

Related: Afterglow

Reference frame

A coordinate system and set of clocks used by an observer to assign positions and times to events.

A reference frame provides the spatial coordinates and clock readings used to describe physical events. In special relativity, inertial frames move at constant velocity relative to one another and are related by Lorentz transformations. Different frames can assign different times, lengths, and simultaneity relations while agreeing on frame-independent physical laws and invariant quantities.

Related: Simultaneity

Refractive index

A measure of how much light slows down in a material relative to vacuum and therefore how strongly its path can bend at a boundary.

The refractive index of a medium is the ratio of the speed of light in vacuum to its speed in that medium. Because refractive index can vary with wavelength, different colors can bend by different amounts in a prism, producing dispersion.

REM sleep

A sleep stage marked by rapid eye movements, characteristic brain activity, and usually reduced skeletal-muscle tone.

REM sleep is a recurring stage of sleep characterized by rapid eye movements, a distinctive pattern of brain activity, and strong suppression of most voluntary muscle activity. Vivid dream reports are especially common after REM awakenings, but dreaming is not exclusive to REM sleep.

Related: Non-REM sleep

Resonance locking

A tidal-evolution mechanism in which a moon's orbital frequency stays coupled to a changing natural oscillation frequency inside its planet.

In resonance locking, the internal structure of an evolving giant planet changes slowly, causing one of its natural oscillation frequencies to drift. A moon can remain near resonance with that mode as its orbit also evolves. The sustained resonance can maintain strong tidal energy transfer and drive orbital migration faster than a model based on a fixed tidal response would predict.

Related: Oscillation mode

Retrotransposon

A mobile genetic element that copies itself through an RNA intermediate that is converted back into DNA.

Retrotransposons are DNA sequences that can make an RNA copy of themselves and then use reverse transcription to generate DNA that is inserted elsewhere in the genome. Their life cycle is one natural example of genetic information flowing from RNA back to DNA.

Ribosomal RNA

RNA that forms a central part of ribosomes, the cellular machinery that builds proteins.

Ribosomal RNA, or rRNA, is a structural and catalytic component of ribosomes. Because some rRNA sequences change slowly over evolutionary time and occur across cellular life, comparisons of them can reveal very deep evolutionary relationships.

Related: Eukaryote

Ribozyme

An RNA molecule that can catalyze a chemical reaction.

Ribozymes are RNA molecules with catalytic activity. Their discovery in the early 1980s overturned the assumption that biological catalysis was exclusive to proteins. Known ribozymes can catalyze reactions such as RNA cleavage, ligation, and processing, and the ribosome's peptide-bond-forming center is itself fundamentally RNA-based.

Related: Catalyst, RNA splicing

RNA splicing

The process that removes introns from a precursor RNA and joins the retained sections together.

During RNA splicing, cellular machinery cuts introns out of a precursor RNA and connects the retained exons. Alternative splicing can join exons in different combinations.

Related: Intron, Exon

Roche limit

The approximate distance from a body inside which tidal forces can prevent loose material held together mainly by self-gravity from assembling into a moon.

The Roche limit compares the tidal forces from a larger body with the self-gravity holding a smaller body or collection of material together. Inside the classical limit, loose material is more easily disrupted or kept from accreting into a moon. The exact distance depends on density, structure, rotation, and material behavior, so it is not a universal hard boundary for every ring system.

Related: Stellar occultation

Schizophrenogenic mother

A discredited mid-20th-century theory that blamed supposedly cold, controlling, or contradictory maternal behavior for causing schizophrenia.

The 'schizophrenogenic mother' was a family-blaming theory influential in parts of mid-20th-century psychiatry. It proposed that particular maternal personality or communication patterns could produce schizophrenia in a child. Later genetic, developmental, epidemiological, and neuroscience research did not support the theory as a causal model.

Related: Double bind

Schwarzschild radius

For an idealized non-rotating, uncharged black hole, the radius of its event horizon. It depends only on the black hole’s mass.

In the Schwarzschild solution of general relativity, the horizon lies at r = 2GM/c². Rotating or electrically charged black holes have different horizon geometry, so “Schwarzschild radius” is not a universal synonym for every black-hole horizon.

Related: Event horizon, General relativity

Sedimentary rock

Rock formed from deposited sediment or from material precipitated from water and later consolidated.

Sedimentary rocks form at or near Earth's surface from accumulated material. Many are produced when fragments of older rocks are deposited, buried, compacted, and cemented; others form when dissolved substances precipitate from water or through biological processes. Their layered structures often preserve evidence about past environments.

Related: Igneous rock, Metamorphic rock, Neptunism

Seven-cell uterus diagram

A medieval anatomical scheme dividing the uterus into seven fetal chambers and assigning different chambers to male, female, or intersex fetuses.

The seven-cell uterus was an inherited medieval anatomical diagram in which the womb was divided into seven compartments, often three on each side plus a central chamber. The cells were linked to theories of fetal sex and bodily heat. Despite human dissection, the scheme persisted in learned anatomy until Renaissance anatomists increasingly replaced it with direct descriptions of a single uterine cavity.

Significant wave height

A standard measure of sea state, historically defined as the average height of the highest one-third of waves.

Significant wave height is used to summarize the size of waves in a sea state. The traditional observational definition is the mean height of the highest one-third of waves; modern instruments often estimate an equivalent quantity statistically from the wave spectrum. Rogue waves are commonly defined relative to this surrounding baseline.

Simultaneity

The relationship of events occurring at the same time; in special relativity, distant events judged simultaneous in one inertial frame need not be simultaneous in another.

Special relativity removes universal simultaneity for spatially separated events. Observers in relative motion use different but internally consistent definitions of which distant events occur at the same time. This relativity of simultaneity follows from the invariance of the speed of light and the Lorentz transformations; it is not caused by inaccurate clocks or signal delays that could simply be corrected away.

Related: Reference frame

STED

A super-resolution method that uses stimulated emission to suppress fluorescence around a small central region, shrinking the effective area that can emit light.

Stimulated emission depletion microscopy, or STED, excites fluorescent molecules and then applies a second, typically doughnut-shaped beam that forces molecules around the edge back to the non-emitting state. Only a much smaller central region remains fluorescent. Scanning that reduced emitting region across the sample produces resolution finer than conventional diffraction-limited imaging.

Related: Super-resolution microscopy, Diffraction limit

Stellar occultation

An event in which a Solar System body or other foreground object passes in front of a star and temporarily blocks some or all of its light.

During a stellar occultation, astronomers record how a background star's brightness changes as a foreground object passes across the line of sight. The timing and shape of the dimming can reveal an object's size, atmosphere, rings, or nearby material even when those features cannot be resolved directly in an image.

Related: Roche limit

STORM

A single-molecule super-resolution method that reconstructs an image from many precisely localized fluorescent emitters switched on in sparse subsets.

Stochastic optical reconstruction microscopy, or STORM, separates nearby fluorescent emitters in time by switching only sparse subsets into an emitting state. Each isolated spot can be localized much more precisely than its diffraction-blurred width. Accumulating many localization events produces a high-resolution reconstruction.

Related: Super-resolution microscopy, PALM, Diffraction limit

Sublunary

In Aristotelian cosmology, the region below the Moon, where ordinary matter was thought to undergo generation, decay, and other kinds of change.

The sublunary realm was the region from Earth up to the Moon in Aristotelian natural philosophy. It was composed of the four terrestrial elements—earth, water, air, and fire—and was the domain of generation, corruption, weather, and rectilinear natural motion. It was contrasted with the supposedly perfect and unchanging celestial realm above the Moon.

Related: Incorruptible, Aether

Super-Kamiokande

A large underground neutrino detector in Japan that uses an enormous tank of ultra-pure water and light sensors to observe neutrino interactions.

Super-Kamiokande is a water-Cherenkov detector located about one kilometer underground in Japan. Charged particles created when neutrinos interact in its water can emit faint Cherenkov light, which thousands of photomultiplier tubes record. Its 1998 atmospheric-neutrino results provided compelling evidence for neutrino oscillation and therefore nonzero neutrino mass.

Related: Neutrino oscillation

Super-resolution microscopy

Optical microscopy methods that recover spatial information finer than the conventional diffraction-limited resolution.

Super-resolution microscopy is a family of techniques that surpass the conventional diffraction-limited resolution of light microscopy by controlling fluorescence or combining information across many measurements. Major approaches include STED, which spatially suppresses fluorescence, and single-molecule localization methods such as PALM and STORM.

Related: Diffraction limit, STED, PALM, STORM

Suppuration

The formation or discharge of pus, usually as part of an inflammatory response to infection or tissue damage.

Suppuration is the production of pus, a fluid containing immune cells, tissue debris, microorganisms, and proteins. Historically, some surgical traditions treated certain kinds of pus as a favorable or even necessary stage of wound healing. Germ theory and antiseptic practice reframed suppuration as something to prevent or control when it reflects infection.

T cell

A type of lymphocyte central to adaptive immunity, with specialized forms that coordinate immune responses, kill infected cells, or regulate other immune cells.

T cells are lymphocytes whose precursors originate in bone marrow but mature in the thymus. Their antigen receptors allow highly specific recognition. Different T-cell populations include helper T cells, cytotoxic T cells, regulatory T cells, and memory cells. Before entering the mature immune system, developing T cells undergo selection in the thymus.

Related: T-cell selection

T-cell selection

The developmental screening of immature T cells in the thymus that preserves useful cells while eliminating or restraining many cells likely to react dangerously against the body's own tissues.

During T-cell development in the thymus, immature cells are tested through several selection steps. Positive selection favors cells whose receptors can interact appropriately with the body's antigen-presenting machinery. Negative selection removes many cells that bind self molecules too strongly, while additional mechanisms generate regulatory cells. Together these processes help produce a useful T-cell repertoire while establishing central immune tolerance.

Related: T cell

Teratogen

An exposure that can disturb embryonic or fetal development and cause congenital abnormalities.

A teratogen is a drug, chemical, infection, physical exposure, or other factor capable of disrupting prenatal development. The effect often depends strongly on dose and on the stage of pregnancy at which exposure occurs.

Tidal heating

Internal heating produced when changing gravitational tides repeatedly flex and deform a moon, planet, or other body.

Tidal heating converts orbital and rotational energy into internal heat through repeated deformation. A moon on a slightly eccentric orbit experiences changing tidal forces as its distance and orientation relative to its planet vary. Internal friction dissipates some of that mechanical energy as heat. The effect powers Io's extreme volcanism and contributes to maintaining subsurface oceans in moons such as Europa and Enceladus.

Related: Cryovolcanism

Transmutation

The conversion of one chemical element into another; in modern physics this requires changing an atomic nucleus.

Transmutation means changing one element into another. Alchemists used the idea in theories about converting base metals into gold through ordinary laboratory operations. Modern chemistry shows that chemical reactions only rearrange electrons and bonds; changing an element requires changing the number of protons in its nucleus through a nuclear process.

Related: Alchemy

Trigeminal system

The sensory nerve system centered on the trigeminal nerve, which carries signals from much of the face, head, and tissues around the brain.

The trigeminal system includes the trigeminal nerve and connected brain pathways that process sensation from the face, scalp, cranial blood vessels, and the protective coverings of the brain. It is a major part of the pain-signaling network involved in migraine.

Vector

A living carrier—often an insect or tick—that transmits a pathogen between hosts.

In infectious-disease ecology, a vector carries a pathogen from one host to another. For urban yellow fever, the principal vector is the mosquito Aedes aegypti.

Related: Fomite

Venera

The Soviet series of Venus probes that achieved the first successful atmospheric entry, landing, surface measurements, and images from another planet.

Venera was the Soviet Union's long-running Venus exploration program. Successive missions measured the planet's dense carbon-dioxide atmosphere, enormous surface pressure, and extreme temperature. Venera 7 made the first successful soft landing on another planet in 1970, and later Venera landers returned surface images and increasingly detailed measurements.

Related: Mariner 2

Weak interaction

One of the four fundamental interactions, responsible for processes including beta decay and neutrino interactions.

The weak interaction is a fundamental interaction of the Standard Model carried by W and Z bosons. It changes particle identities in processes such as radioactive beta decay and governs many neutrino reactions. Unlike electromagnetism and the strong interaction, weak processes can violate parity symmetry.

Related: Parity, Neutrino flavor

Yamnaya

Mobile pastoralist communities of the Pontic-Caspian steppe whose ancestry spread widely into Europe during the third millennium BCE.

Yamnaya refers to an archaeological horizon of pastoralist communities centered on the Pontic-Caspian steppe around the late fourth and early third millennia BCE. Ancient human DNA shows a major spread of steppe-related ancestry into large parts of Europe during this period. Archaeological evidence also includes distinctive burial mounds, pastoral economies, and wheeled transport.

Related: Ancient DNA