It is basically feeling that you are not really a successful, competent, and smart student, that you are only posing as such.
Some common feelings and thoughts that might characterize the impostor syndrome are: “I feel like a fake” “My classmates/professors etc. are going to find out I don’t really belong here,” “Admissions made a mistake”
I successfully defended my dissertation today. Most importantly, this was my outfit (except I had brown pants. And no earrings. And heels! Imagine! I didn't even trip). Seriously, wouldn't you have passed me? It was grand--a full house, cookies, good questions.
Now for some serious wall-staring. Short sentences that don't cite any authoritative source. Playing with trucks and blocks. I totally did it.