The Collaborative International Dictionary of English v.0.48:

Pseudonym \Pseu"do*nym\, n. [Cf. F. pseudonyme. See
   Pseudonymous.]
   A fictitious name assumed for the time, as by an author; a
   pen name; an alias. [Written also pseudonyme.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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