Something that very few people know the true meaning of. Even though deja vu is French for "already seen", it actually is used to describe the strange feeling you get when you're in a situation, and feel like you've been in the exact same situation before, but really haven't. Most people think it means the same thing happening twice, which is wrong. Get it right people!
Jack: I could have sworn I've been here before. But I know I haven't.
Bob: Heh. Deja vu?
Jack. Yeah. What a strange feeling!