It is a chemical required for some bodily function, often involving metabolism, that cannot be made by us. Vitamins function as catalysts in chemical reactions in the body. They speed up metabolism but do not get used up.
Most vitamins serve multiple functions.
All of these can either be synthesized in the body or are not required. Although vitamin deficiency diseases are described in writings thousands of years old, the term vitamin was coined in 1911 when the first was isolated and its essential properties demonstrated. Before, most deficiencies were thought due to infections or other causes. The first half of the 20th century saw the discovery of all vitamins and most essential nutrients. Zinc was last to be identified as essential for humans in 1963.