Déjà vu is French for "seen before." Today, the term déjà vu is used to describe a new experience that feels as if it's not new. For example, if you travel someplace you've never been before, and while you're walking down the street you suddenly feel as if you've been in that very place before, it's déjà vu. Arthur Funkhouser, a Swiss scholar, maintains that there are different types of déjà vu. For example, déjà visite, "already visited," would be the type of déjà vu described above. In contrast to this, if you went out with friends and started discussing current events, and then suddenly felt that you'd already had that conversation with those very people, in that very place, it would be called déjà vecu, "already lived through" [source: Funkhouser].
About 60 to 70 percent of people will experience déjà vu at least once in their lives, and for some reason, it seems to affect people between the ages of 15 and 25 even more than others [source: Brown]. No one is completely sure of what causes déjà vu -- there are more than 40 different theories floating around -- but people attribute it to everything from glitches in memory to reincarnation.
Most who have researched the subject agree that déjà vu falls into two categories: associative déjà vu and biological déjà vu. Associative déjà vu is the most common type and the one that most healthy people experience. Biological déjà vu, on the other hand, tends to be tied to temporal lobe epilepsy. Right before someone with the condition has a seizure, he may experience a very strong feeling of déjà vu, sometimes truly believing that he's been through that exact moment before.
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