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]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "impostor":
   ape, beguiler, blagueur, bluff, bluffer, charlatan, cheat,
   clinquant, con man, confidence man, conformist, copier, copycat,
   copyist, counterfeit, counterfeiter, cuckoo, deceiver, dissembler,
   dissimulator, dummy, echo, echoer, echoist, fake, fakement, faker,
   flimflam man, forger, forgery, fourflusher, frame-up, fraud, hoax,
   humbug, hypocrite, imitation, imitator, impersonator, imposter,
   junk, malingerer, mime, mimer, mimic, mimicker, misleader, mock,
   mocker, mockingbird, monkey, mountebank, parrot, paste,
   pettifogger, phony, pinchbeck, plagiarist, poll-parrot, polly,
   polly-parrot, poser, poseur, pretender, put-on, put-up job, quack,
   quacksalver, quackster, ringer, rip-off, saltimbanco, sham,
   shammer, shark, sheep, shoddy, shyster, simulacrum, simulator,
   swindle, swindler, tinsel, trickster, whited sepulcher



The Devil's Dictionary (1881-1906):

IMPOSTOR n.  A rival aspirant to public honors.