A sense of anxiety/dread/fear that you’re not as good at something as you are, and that any moment it’s all going to come crashing down. For example fear your boss will see you’re a moron and your work sucks, even though there’s no evidence that’s the case. And in fact there’s evidence to the contrary; you’ve been at your job for years, you have good reviews, you’ve been promoted. Fear that everyone in the conference room will see through you and know that you’re a fraud, that you don’t really know what you’re talking about, that you don’t deserve to be there. Even though you’ve given successful presentations dozens of times.

Also it should be noted that occasionally feeling this way is perfectly normal.