A chemical reaction in which substances combine with oxygen for form an oxide. For example, the combination of iron with oxygen to form an iron oxide (rust) or copper and oxygen produce copper oxide; the green coating on old pennies. The opposite of oxidation is reduction.
In geological terms oxidation on iron-bearing silicate minerals causing "rusting" of the minerals, for example, oxidation of olivine, pyroxenes, amphiboles, and biotite produces limonite, hematite, and goethite. The end result is an iron-rich rock, such as a basalt weathers to a red-orange colour.
Salisbury Crags (dolerite: a basic igneous rock), Edinburgh, showing classic "rusting". Image courtesy of Tr1xx.