(1) The combination of oxygen with a substance forming oxide.
(2) A chemical reaction in which there is the loss of electrons or gain (or increase in the proportion) of oxygen, hence, resulting in an increase in oxidation state by a molecule, atom or ion.
(3) A process resulting in an increase in oxidation number.
For instance in cellular respiration, the oxidation of glucose (C6H12O6) produces CO2.
Word origin: French, from oxider, to oxidize, from oxide, oxide.
Related forms: oxidative (adjective), oxidatively (adverb).
See also: redox reaction