The Collaborative International Dictionary of English v.0.48:

Impersonator \Im*per"son*a`tor\, n.
   One who impersonates; an actor; a mimic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impersonator
    n 1: someone who (fraudulently) assumes the appearance of
         another [syn: impersonator, imitator]