When something oxidizes, it joins with oxygen in the air, and that change often shows on the surface. Iron oxidizes and we call the brown, rough coat rust; leave a bike in the rain long enough and the chain can even flake. A cut apple oxidizes too — the white face goes brown. Copper oxidizes and slowly turns green. You can also oxidize something on purpose, by heat or by a chemical, so the word covers both “it happens by itself” and “someone makes it happen.” Chemists use it more widely, though actually: for them it names a bigger family of changes, and oxygen from the air is not always in the story. Rust is only the picture most of us know. In everyday talk, people still mean “it met oxygen and changed,” especially metal going rusty. American spelling is oxidize; British spelling is often oxidise. The name of the change itself is oxidation.