An oxidation reaction is one in which oxygen combines with an element or a compound.
Oxidation is the loss of electrons during a reaction by a molecule, atom or ion. Oxidation occurs when the oxidation state of a molecule, atom or ion is increased.
A classic example of the old definition of oxidation is when iron combines with oxygen to form iron oxide or rust. The iron is said to have oxidized into rust. The chemical reaction is:
2Fe + O2 → Fe2O3