The Collaborative International Dictionary of English v.0.48:

Postfix \Post"fix\, n.; pl. Postfixes. [Pref. post- + -fix, as
   in prefix: cf. F. postfixe.] (Gram.)
   A letter, syllable, or word, added to the end of another
   word; a suffix. --Parkhurst.
   [1913 Webster]