First things first, it's really hard to define déjà vu scientifically.
Most people probably understand what someone means when they say they've experienced déjà vu. In French, it literally means "already seen". But in order to study the phenomenon, scientists need to be more specific.
So we asked Dr Akira O'Connor, a researcher specialising in memory at the University of St Andrews. He told BuzzFeed Science over email that a common definition comes from psychologist Alan Brown and can be paraphrased like this:
Déjà vu is a subjective experience of familiarity, alongside an objective experience of unfamiliarity.
"That is, you know something shouldn't feel as familiar to you as it currently feels," says O'Connor.
Around 60% of people report experiencing déjà vu at some point.
"There are a bunch of different ideas about what might cause déjà vu but the jury is very much still out on a definitive answer," says O'Connor.
There's some evidence a brain chemical called dopamine could be involved.
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Kelly Oakes is science editor for BuzzFeed and is based in London.
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