Source : Webster's Revised Unabridged Dictionary (1913)

Toxin \Tox"in\, Toxine \Tox"ine\, n. [Gr. toxiko`n. See {Toxic}.
   ]
   A poisonous product formed by pathogenic bacteria

Source : WordNet®

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