nom : Idioms & Phrases

• noun a fictitious name used when the person performs a particular social role
  anonym; pseudonym.
  

WordNet

• literally, war name; hence, a fictitious name, or one assumed for a time.

Webster 1913

• noun an author's pseudonym
  pen name.
  

WordNet

• literally, pen name; hence, a name assumed by an author as his or her signature.

Webster 1913