The Collaborative International Dictionary of English v.0.48:

Anonym \An"o*nym\, n. [F. anonyme. See Anonymous.]
   1. One who is anonymous; also sometimes used for "pseudonym."
      [1913 Webster]

   2. A notion which has no name, or which can not be expressed
      by a single English word. [R.] --J. R. Seeley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

anonym
    n 1: a fictitious name used when the person performs a
         particular social role [syn: pseudonym, anonym, nom de
         guerre]