Any of a group of organic substances essential in small quantities to normal metabolism WordNet 3.0 & Open English WordNet

Vitamins are organic molecules that are essential to an organism in small quantities for proper metabolic function. Wikipedia

Organic compound and a vital nutrient that an organism requires in limited amounts. Wikipedia Disambiguation

An organic compound present in variable, minute quantities in natural foodstuffs and essential for the normal processes of growth and maintenance of the body. OmegaWiki

Any of a specific group of organic compounds essential in small quantities for healthy human growth, metabolism, development, and body function; found in minute amounts in plant and animal foods or sometimes produced synthetically; deficiencies of specific vitamins produce specific disorders. Wiktionary

Organic compound essential to human health. Wiktionary (translation)

(attributive use of the noun). Wiktionary (translation)