Vitamins are organic molecules (or a set of closely related molecules called vitamers) that are essential to an organism in small quantities for proper metabolic function. Essential nutrients cannot be synthesized in the organism in sufficient quantities for survival, and therefore must be obtained through the diet. For example, Vitamin C can be synthesized by some species but not by others; it is not considered a vitamin in the first instance but is in the second. Most vitamins are not single molecules, but groups of related molecules called vitamers. For example, there are eight vitamers of vitamin E: four tocopherols and four tocotrienols.

The term vitamin does not include the three other groups of essential nutrients: minerals, essential fatty acids, and essential amino acids
Vitamin A (as all-trans-retinol, all-trans-retinyl-esters, as well as all-trans-beta-carotene and other provitamin A carotenoids)
Vitamin B1 (thiamine)
Vitamin B2 (riboflavin)
Vitamin B3 (niacin)
Vitamin B5 (pantothenic acid)
Vitamin B6 (pyridoxine)
Vitamin B7 (biotin)
Vitamin B9 (folic acid or folate)
Vitamin B12 (cobalamins)
Vitamin C (ascorbic acid)
Vitamin D (calciferols)
Vitamin E (tocopherols and tocotrienols)
vitamin K (phylloquinone and menaquinones