Antioxidant, an tee OK suh duhnt, is any of a group of chemical compounds that may prevent certain types of cell damage. Antioxidants block the effects of oxidation, a chemical reaction in which a substance loses electrons, often while combining with oxygen. Antioxidants are important because they protect cells from the effects of free radicals, unstable molecules produced by oxidation. Scientists believe free radicals may be involved in the aging process as well as in a number of diseases.