The Collaborative International Dictionary of English v.0.48:

coenzyme \co*en"zyme\ (k[-o]*[e^]n"z[imac]m), (Biochem.)
   a molecule that is essential for the activity of some
   enzymes; it may exist free in solution within a living
   organism, but functions by binding to an enzyme to assist in
   catalyzing a reaction. The molecule itself may be temporarily
   changed during the reaction, but is ultimately restored to
   its original form. Many vitamins function as coenzymes.
   [PJC]

WordNet (r) 3.0 (2006):

coenzyme
    n 1: a small molecule (not a protein but sometimes a vitamin)
         essential for the activity of some enzymes