adulterant

from WordNet (r) 3.0 (2006)

adulterant
    adj 1: making impure or corrupt by adding extraneous materials;
           "the adulterating effect of extraneous materials" [syn:
           {adulterating}, {adulterant}] [ant: {purifying}]
    n 1: any substance that lessens the purity or effectiveness of a
         substance; "it is necessary to remove the adulterants
         before use" [syn: {adulterant}, {adulterator}]

from The Collaborative International Dictionary of English v.0.48

Adulterant \A*dul"ter*ant\, n. [L. adulterans, p. pr. of
   adulterare.]
   That which is used to adulterate anything. -- a.
   Adulterating; as, adulterant agents and processes.
   [1913 Webster]