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.