The Collaborative International Dictionary of English v.0.48:

Impostor \Im*pos"tor\, n. [L. impostor a deceiver, fr. imponere
   to impose upon, deceive. See Impone.]
   One who imposes upon others; a person who assumes a character
   or title not his own, for the purpose of deception; a
   pretender. "The fraudulent impostor foul." --Milton.

   Syn: Deceiver; cheat; rogue. See Deceiver.
        [1913 Webster]

WordNet (r) 3.0 (2006):

impostor
    n 1: a person who makes deceitful pretenses [syn: imposter,
         impostor, pretender, fake, faker, fraud, sham,
         shammer, pseudo, pseud, role player]

The Devil's Dictionary (1881-1906):

IMPOSTOR n.  A rival aspirant to public honors.