The Collaborative International Dictionary of English v.0.48:

enzyme \en"zyme\ ([e^]n"z[imac]m), n. [Pref. en- (Gr. 'en in) +
   Gr. zy`mh leaven.] (Physiol. Chem.)
   A protein produced by a living organism, capable of
   catalyzing a chemical reaction. Almost all processes in
   living organisms require some form of enzyme to cause the
   reactions to occur at a rate sufficient to support life.
   There are a very wide variety of enzymes, each specifically
   catalyzing a different chemical reaction, the sum of which
   cause the bulk of the physiological changes observed as life
   processes. Enzymes, like most proteins, are synthesized by
   the protein-synthetic mechanism of the living cell, at
   special sites on ribosomes, using the genetic information in
   messenger RNA transcribed from the genetic instructions
   stored as nuleotide sequences in the DNA (or in some viruses,
   the RNA) of the genome. Some examples of enzymes are: pepsin,
   diastase, rennet, DNA polymerase, invertase, glucose oxidase,
   protease, and ribonuclease. There are many other types of
   enzyme.
   [1913 Webster +PJC]

   Note: The 1913 Webster defined an enzyme as:
         An unorganized or unformed ferment, in distinction from
         an organized or living ferment; a soluble, or chemical,
         ferment.
         [PJC]

WordNet (r) 3.0 (2006):

enzyme
    n 1: any of several complex proteins that are produced by cells
         and act as catalysts in specific biochemical reactions

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "enzyme":
   beta-amylase, carbohydrase, coenzyme, deoxyribonuclease, digestant,
   digester, digestive, esterase, holoenzyme, insulinase, lipase,
   lipoxidase, pepsin, polynucleotidase, protease, proteolytic enzyme,
   ribonuclease, saccharase, saccharifying enzyme