The feeling that you are doing something that you've already done before. Literal translation "already seen" (from French, deja 'already', vu 'seen'). Apparently it's due to a blip in your brain process which gives you the illusion that you've been somewhere/done something before, and technically, you have - a fraction of a second ago.
However, a common misinterpretation of deja vu is a repeat of a certain event. For example, in an episode of
Garfield and Friends (US Acres) Orson perpetually removes the same book (Deja vu: the feeling that you are doing something that you've done before) from a box of books
over and over again. This is NOT deja vu; rather, it's an event that doesn't have a forseeable end: an
infinite loop, if you will.
If you walk into a room and immediately get the feeling that you've been there before, then that is a case of deja vu.