One who engages in deception under an assumed name or identity.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "impostor" in a sentence
"The case was the first of its kind for the Bay City office, Flattery said, but the medal-wearing impostor is not uncommon."
"Phony heroes have been around as long as wars, and one way to spot an impostor is to listen to their exaggerated exploits."
"The Schantags issue "fraud alerts" through e-mail when an impostor is found."