Imposture is the act of pretending to be someone else. Everyone knows the Elvis impersonator isn’t really Elvis himself, but your imposture as Elvis’s long-lost daughter might actually fool some people.
Imposture comes from the verb, to impose, and it has the sense of deliberately deceiving someone. Someone who perpetrates an imposture is an imposter. If you go to a job interview and pretend that you graduated from Harvard when really you never even went to college, that is an act of imposture. If the interviewer finds out, she might disgustedly say to you, “Get out, you imposter!”