im·pos·tor or im·pos·ter 
(ĭm-pŏstər)
(ĭm-pŏstər)n.
One who engages in deception under an assumed name or identity.
[French imposteur, from Latin impostor, one who assigns a name, from impostus, variant of impositus, past participle of impōnere, to place upon; see IMPOSE.]