The Collaborative International Dictionary of English v.0.48:

Imposture \Im*pos"ture\, n. [L. impostura: cf. F. imposture. See
   Impone.]
   The act or conduct of an impostor; deception practiced under
   a false or assumed character; fraud or imposition; cheating.
   [1913 Webster]

         From new legends
         And fill the world with follies and impostures.
                                                  --Johnson.

   Syn: Cheat; fraud; trick; imposition; delusion.
        [1913 Webster]

WordNet (r) 3.0 (2006):

imposture
    n 1: pretending to be another person [syn: imposture,
         impersonation]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "imposture":
   acting, affectation, appearance, artifice, attitudinizing,
   ballot-box stuffing, bluff, bluffing, bunco, cardsharping,
   charlatanism, charlatanry, cheat, cheating, color, coloring, copy,
   copying, counterfeit, counterfeiting, cozenage, deceit, deception,
   delusion, diddle, diddling, disguise, dishonesty, dissemblance,
   dissembling, dissimulation, dodge, emulation, fabrication, facade,
   face, fake, fakery, faking, false air, false front, false show,
   falsity, feigning, feint, fishy transaction, flam, flimflam,
   following, forgery, four-flushing, fraud, fraudulence, fraudulency,
   front, gambit, gerrymandering, gilt, gloss, graft, grift, gyp,
   gyp joint, hit-off, hoax, humbug, humbuggery, illicit business,
   imitation, impersonation, imposition, impression, make-believe,
   maneuver, masquerade, meretriciousness, mimesis, mirroring,
   onomatopoeia, ostentation, outward show, parody, phony, plagiarism,
   plagiary, playacting, ploy, pose, posing, posture, pretense,
   pretension, pretext, put-on, quackery, quackishness, quackism,
   racket, repetition, representation, ruse, scam, seeming, semblance,
   sham, show, simulacrum, simulation, sleight, speciousness, spoof,
   swindle, takeoff, varnish, wile, window dressing