Oxidation always means adding oxygen to a substance.

The word oxidation originally meant what its name suggests: a reaction involving oxygen.

That definition worked well for familiar examples. Iron rusts by reacting with oxygen, and combustion often involves a substance combining with oxygen from the air.

Chemists later found reactions that behaved in the same way chemically even though no oxygen was present.

The broader pattern became clear with electron theory. In an oxidation reaction, a substance loses electrons. The partner reaction is reduction, in which another substance gains electrons. Because the two always occur together, chemists often call them redox reactions.

For example, chlorine can oxidize a metal by taking electrons from it even though the reaction contains no oxygen at all.

Chemists also describe oxidation using oxidation states, a bookkeeping system for tracking how electrons are distributed in compounds. When an atom’s oxidation state increases, it has been oxidized.

Oxygen is still a very common oxidizing agent, so the old definition remains useful for many everyday reactions. What changed was the general rule: oxygen is one way to cause oxidation, not what defines oxidation itself.