This is an interesting topic of psychological research. Basically, it is the feeling that despite your accomplishments you feel like you are faking it. I think that is more common in women and minorities in a field, and what prompted this post was a very accomplished woman of my acquaintance mentioning it.
I have a theory that it might also be more common in computing circles. It is hard to admit that you don’t know something (see previous posts) and the atmosphere in the computing community may make that harder. There was a very influential paper that discusses academic computing communities (let’s not even get into some other computing communities!) called ‘Defensive climate in the computer science classroom’ by Barker, Garvin-Doxas and Jackson that discusses this – you can search for it on-line.
It is good for us, in every way, to find friends who are supportive and who will understand our accomplishments realistically – so that when we have moments of self-doubt we can get a bit of cheerleading, and give the same in return.