The Collaborative International Dictionary of English v.0.48:

Toxoid \Tox"oid\, n. [Toxin + -oid.] (Physiol. Chem.)
   An altered form of a toxin, possessing little or no toxic
   power.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

toxoid
    n 1: a bacterial toxin that has been weakened until it is no
         longer toxic but is strong enough to induce the formation
         of antibodies and immunity to the specific disease caused
         by the toxin; "diphtheria toxoid" [syn: anatoxin,
         toxoid]