Cellular mechanisms that prevent free radical damage. An antioxidant is any substance that delays, prevents or removes oxidative damage to a target molecule. This concept includes, among others, protein enzymatic activities, metal chelators as well as peptide and non-peptide small molecules.
Antioxidants neutralize free radicals by accepting or donating electron(s) to eliminate the unpaired condition of the radical. The antioxidant molecules may directly react with the reactive radicals and destroy them, or they may themselves become new free radicals which are less active, longer-lived and less dangerous than those radicals they have neutralized.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2927345/
Free Radicals in Biology and Medicine. Fifth edition. Barry Halliwell and John M.C. Gutteridge. Oxford University Press 2015.