A vitamin is a chemical substance required, in very small amounts, to maintain an organism's health.They act as a catalyst in the body's chemical reaction. It cannot be produced by the organism, and so must be obtained from dietary or other sources. In order for a substance to be classified as a vitamin, its deficiency must produce a specific disease. For example, ascorbic acid is a vitamin (Vitamin C) in humans because its absence produces scurvy, and thiamine is a vitamin (Vitamin B1) because its absence produces beriberi. There are exactly thirteen known human vitamins.
Some substances are vitamins in certain organisms, but not in others (for example, ascorbic acid can be produced by all mammals except guinea pigs and humans, and so is a vitamin only in those two instances.