Someone who pretends to be someone else in order to deceive others.
Imposter has a sneaky and dishonest feeling.
Examples:
The imposter pretended to be a doctor.
She knew he was an imposter because he didn't know anything about the job.
The imposter tried to steal the king's crown.
The imposter said he could fly, but he couldn't even jump high!
The imposter claimed to be from another planet.
The imposter was caught and put in jail.