A vitamin is an organic molecule (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.

Major health organizations list thirteen vitamins:

• 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)

Some sources include a fourteenth, choline.

Vitamins have diverse biochemical functions. Vitamin A acts as a regulator of cell and tissue growth and differentiation. Vitamin D provides a hormone-like function, regulating mineral metabolism for bones and other organs. The B complex vitamins function as enzyme cofactors (coenzymes) or the precursors for them. Vitamins C and E function as antioxidants. Both deficient and excess intake of a vitamin can potentially cause clinically significant illness, although excess intake of water-soluble vitamins is less likely to do so.

All the vitamins were discovered between 1913 and 1948. Historically, when intake of vitamins from diet was lacking, the results were vitamin deficiency diseases. Then, starting in 1935, commercially produced tablets of yeast-extract vitamin B complex and semi-synthetic vitamin C became available. This was followed in the 1950s by the mass production and marketing of vitamin supplements, including multivitamins, to prevent vitamin deficiencies in the general population. Governments have mandated the addition of some vitamins to staple foods such as flour or milk, referred to as food fortification, to prevent deficiencies. Recommendations for folic acid supplementation during pregnancy reduced risk of infant neural tube defects.