An antioxidant is a molecule that inhibits the oxidation of other molecules.
Oxidation is a chemical reaction that can produce free radicals, leading to chain reactions that may damage cells. Antioxidants such as thiols or ascorbic acid (vitamin C) terminate these chain reactions.

To balance the oxidative state, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione and enzymes (e.g., catalase and superoxide dismutase) produced internally or the dietary antioxidants, vitamin A, vitamin C, and vitamin E.

Diets containing foods rich in antioxidant vitamins or antioxidant dietary supplements do not improve health nor are they effective in preventing diseases. Randomized clinical trials including supplements of beta-carotene, vitamin A, and vitamin E singly or in different combinations found no effect on mortality rate and cancer risk, or were found to may even increase cancer risk.
Supplementation with selenium or vitamin E does not reduce the risk of cardiovascular disease.
Oxidative stress can be considered as either a cause or consequence of some diseases, an area of research stimulating drug development for antioxidant compounds for use as potential therapies.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on January 18, 2016.