'Déjà vu' is a french term that literally means "already seen" and has several variations, including 'déjà vécu', already experienced; 'déjà senti', already thought; and 'déjà visité', already visited. French scientist Emile Boirac, one of the first to study this strange phenomenon, gave the subject its name in 1876.
Trivia: Research shows that almost 70% of people experience 'deja vu' at least once before the age of 25 and that it is usually triggered not by big events but by life's mundane details (like the pattern of the dishes stacked in the kitchen or the window-display in a shop).
My favourite theory explaining 'deja vu' is from the movie "The Matrix" :)
NEO: A black cat went past us and then I saw another that looked just like it.
TRINITY: How much like it? Was it the same cat?
NEO: It might have been. I'm not sure. What is it?
TRINITY: A deja vu is usually a glitch in the Matrix. It happens when they change something.