Imagine a lawyer who is terrified to go to work every day. She went to a good law school, got good grades, passed the bar exam, is well-respected in her field, etc. But she is terrified that her colleagues will, one of these days, find out that she's not even truly a lawyer at all -- she's just someone who managed to jump through enough hoops to convince people that she's a real lawyer. Once the people around her learn who she really is, hoo boy... she's going to lose her law license and her reputation and her clients and her house... she lives in fear of being "found out" every single day and it's beginning to affect both her career and her health.
Is this lawyer an imposter? No, she's truly the lawyer everyone thinks she is, and it would be inaccurate to call her an imposter. But she is suffering from imposter syndrome.