means A chemical reaction in which a substance loses electrons, often by combining with oxygen, producing things like rust, tarnish, and the browning of a cut apple.

from Built from "oxygen" plus the action-suffix "-ation." The story starts with oxygen itself, named in the late 18th century by the French chemist Lavoisier from Greek roots meaning roughly "acid-maker" (oxys, "sharp" or "acid," plus a generator suffix) — because he mistakenly believed oxygen was the essential ingredient in all acids. He was wrong about the acids, but the name stuck, and "oxidation" rode out from it to describe any reaction in that family. Chemists later realized the process is really about losing electrons, so "oxidation" now covers plenty of reactions where no oxygen appears at all — a word that outgrew its own etymology.