Feeling like a fraud or impostor is not a "syndrome," but it's a not-uncommon feeling, and sometimes it's a symptom.
Sometimes feeling fraudulent it can be a reflection of reality: many do not feel that they know all they should, or have all of the skills they believe they should have, to present themselves as sufficiently expert in something - and they might be right. That's the point Right Wing Prof is making in his piece about the "impostor syndrome" in academics. Feeling like an impostor can be simply the result of a forceful self-critique. For example: "Here I am applying for a job teaching literature, and I can't remember a darn thing about Beowulf."
Similarly, many people puff themselves up, polish their presentations to the world, to conceal their flaws and weaknesses and to exaggerate their strengths. It often makes sense in life to do so - to put one's best foot forward - but at some point it can also leave a person prone to feeling that his life is an act or a sham, with only the illusion of substance or authenticity. For examples, "I know I'm a coward, but I need the world to see me as brave," or "I know I'm no genius, but I need people to think I'm a sophisticated intellectual." (Related: our recent post on masquerades and Social Signaling)
Getting one's confidence in line with one's personal reality, one's potential, and one's achievements, without false humility and without false advertising, is not always easy.
There are also more neurotic reasons for a person to have, as a symptom, a feeling of fraudulence, such as masochistic self-doubt or failed narcissism, but I won't go into those right now.
