Déjà vu (literally, French for “already seen”) is the feeling that we have already seen or experienced something that we are seeing or experiencing for the first time.  Occasional déjà vu experiences are normal, and they may reflect momentary glitches in memory. One explanation is that the neurons in the brain may get temporarily out of phase, and mislabel incoming new information as old.  This may be particularly likely if some elements of the current situation are familiar, so that some aspects of the current situation match an old situation.  Another explanation for déjà vu is a momentary lapse in attention:  you see or hear something but aren’t paying attention and, a moment later, you process the scene more fully and think it’s familiar – and it really is, because you processed it a moment ago.

The brain basis of déjà vu is not understood, but it may involve malfunction of the temporal lobes, a part of the brain that is important for new memory formation. Some patients, with damage to the temporal lobes or with epilepsy that originates in the temporal lobes,  experience persistent déjà vu.

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Article : "Been There, Done That, Seen That"