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]

The Collaborative International Dictionary of English v.0.48:

Postfix \Post*fix"\, v. t.
   To annex; specifically (Gram.), to add or annex, as a letter,
   syllable, or word, to the end of another or principal word;
   to suffix. --Parkhurst.
   [1913 Webster]

WordNet (r) 3.0 (2006):

postfix
    n 1: an affix that is added at the end of the word [syn:
         suffix, postfix]