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. ^[2]

Major health organizations list thirteen vitamins:

• Vitamin A (all- trans -retinols, all- trans -retinyl-esters, as well as all- trans -beta-carotene and other provitamin A carotenoids)
• Vitamin B ₁  (thiamine)
• Vitamin B ₂  (riboflavin)
• Vitamin B ₃  (niacin)
• Vitamin B ₅  (pantothenic acid)
• Vitamin B ₆  (pyridoxine)
• Vitamin B ₇  (biotin)
• Vitamin B ₉  (folic acid and folates)
• Vitamin B ₁₂  (cobalamins)
• Vitamin C (ascorbic acid and ascorbates)
• Vitamin D (calciferols)
• Vitamin E (tocopherols and tocotrienols)
• Vitamin K (phylloquinones, menaquinones, and menadiones)

Some sources include a fourteenth, choline.