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Déjà vu is a quirk of memory in which a new experience seems familiar.
This phenomenon of the mind is extremely common, but its nature makes it difficult to test in an empirical setting. Accordingly, there has been very little solid scientific research done on déjà vu, and neither its trigger nor its mechanism is fully understood.
Déjà vu has often been linked with various kinds of woo because of its ubiquitous yet mysterious nature: nearly everyone experiences the phenomenon, but no one can satisfactorily explain either what causes it or how it works. This allows for all kinds of conjectures about experiences in "past lives" or a psychic ability to "remember" the future. (Insert rolled eyes here.)
One of the most popular fictional explanations of déjà vu occurred in the hit 1999 film The Matrix, where it is explained as programming glitches in the simulated reality.
Often described as the counterpart or "opposite" to déjà vu, this is the phenomenon in which an often-repeated experience feels as if it is new to the observer. The most common example can be induced by repeatedly saying a common word - "girls" is often used in this exercise - many times in a row. At some point in the repetition, most people find that the word ceases to make immediate sense to them as a communicating symbol, leaving them free to experience the word as wholly new before the effect fades.