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

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "toxin":
   abomination, acaricide, anthelmintic, antibiotic, antiseptic,
   atrocity, bad, bane, befoulment, blight, bug bomb,
   carbamate insecticide, chemosterilant,
   chlorinated hydrocarbon insecticide, contact poison, corruption,
   crying evil, damage, defilement, defoliant, despoliation,
   destruction, detriment, disinfectant, eradicant, evil, fumigant,
   fungicide, germicide, grievance, harm, havoc, herbicide, hurt, ill,
   infection, injury, insect powder, insecticide, microbicide,
   mischief, miticide, organic chlorine,
   organic phosphate insecticide, outrage, pesticide, poison,
   pollution, rat poison, roach paste, roach powder, rodenticide,
   stomach poison, systemic, systemic insecticide, the worst, toxic,
   toxicant, venin, venom, vermicide, vexation, virus, weed killer,
   woe, wrong