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]