We've all had it. Probably. Okay it's actually more like 70% of the population. That's still pretty considerable when you think about it.
70% of people have experienced the phenomenon known as Déjà Vu, which literally translates from French to "already seen". Déjà Vu is that strange feeling that you get when you think that you've experienced something already, and not in the "if-I-sit-through-grandma-telling-this-story-one-more-time-I-swear" sort of way. This is when something happens for the first time and it almost feels as if you could recall it from your memory, despite you never having experienced it before.
Arthur Funkhouser, who, aside from having a pretty cool name is a Swiss scholar, describes two different categories of Déjà Vu. Déjà Visite ("already visited") describes, as you would imagine, the feeling that you have been to a certain place before, even though you haven't. Déjà Vecu ("already experienced or lived through") refers to the the feeling that something has already happened to you when it hasn't.
Déjà Vu has been the cause of much metaphorical head scratching over the years and there is still a degree of mystery that surrounds it. What we do know, at this point, is that Déjà Vu is linked with temporal-lobe epilepsy. This, of course, does not imply that everyone who has ever experienced Déjà Vu is epileptic. What this does mean, however, is that many people who experience these sorts of epileptic seizures may experience the feeling of Déjà Vu during or between episodes.
Another thing we know about Déjà Vu is that it is most prevalent between the ages of 15-25 years old. The direct cause of Déjà Vu is not explicitly known yet. Many theories exist on the subject, among them being the idea that they are fueled by fantasy or wish fulfillment or perhaps a malfunction in the brain causing it to confuse the present with the past. Parapsychologists would have you believe that Déjà Vu is linked to past lives, but there is no evidence to support this idea.