The Collaborative International Dictionary of English v.0.48:

Toxin \Tox"in\, Toxine \Tox"ine\, n. [Gr. toxiko`n. See Toxic.
   ]
   A poisonous product formed by an organism, such as a
   pathogenic bacterium, a plant or an animal, usually having a
   high molecular weight, often a protein or a polysaccharide,
   but occasionally a low-molecular weight agent such as
   tetrodotoxin.

WordNet (r) 3.0 (2006):

toxin
    n 1: a poisonous substance produced during the metabolism and
         growth of certain microorganisms and some higher plant and
         animal species