The Collaborative International Dictionary of English v.0.48:

Vitamin \Vit"a*min\ (v[imac]t"[.a]*m[i^]n), n.
   any of several organic chemical substances not synthesized by
   an animal and required in small quantities for normal
   metabolism, present in and obtained from the natural foods
   eaten by the animal. Human vitamins are also produced
   synthetically, and taken in pure form or in mixtures, as
   dietary supplements. Deficiencies of specific vitamins lead
   to certain specific disorders, such as scurvy, caused by an
   insufficiency of vitamin C (ascorbic acid).

   Note: Most vitamins act as coenzymes or precursors to
         coenzymes, and are not consumed for energy production
         or incorporated into structural units of the cell.
         [PJC]

WordNet (r) 3.0 (2006):

vitamin
    n 1: any of a group of organic substances essential in small
         quantities to normal metabolism

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "vitamin":
   adermin, adjuvant, aneurin, antiberi-beri factor, ascorbic acid,
   axerophthol, biotin, carminative, carotene, cholecalciferol,
   choline, cobalamin, counterirritant, cryptoxanthin, cyanocobalamin,
   emmenagogue, ergocalciferol, expectorant, folic acid, hepatoflavin,
   hormone, inositol, lactoflavin, maturative, menadione,
   naphthoquinone, niacin, nicotinic acid, ovoflavin, pyridoxine,
   tocopherol, vasodilator, vitamin B, vitamin Bc, vitamin D,
   vitamin G, vitamin H, vitamin K, vitamin M