A vitamin is an organic compound required by an organism as a vital nutrient in limited amounts. An organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on the circumstances and on the particular organism. For example, ascorbic acid is a vitamin for humans, but not for most other animals, and biotin and vitamin D are required in the human diet only in certain circumstances.