Oxidation is the process of combining with oxygen, or of making something combine with oxygen. The picture everyone knows is rust: iron meets air and water, joins with oxygen, and the metal goes brown and weak. Chemists use the word more widely — not only rust, but many other “joining with oxygen” changes, and later in school science it also covers related changes that do not always look like rust at all. You hear it in science class, in talk about metal going bad, and sometimes about food or old paint. The everyday noun is oxidation (the process); the verb is oxidize; the stuff you often get is an oxide. It is a school-and-science word, not a kitchen-table word, though the rust example is something everyone has seen. Though actually rust is only oxidation of iron; a sliced apple turning brown is a kind of oxidation too.