The French language has gifted quite a few phrases and expressions to English, but often within France itself these phrases have a slightly different meaning (like voilà or oh là là) or are simply not used any more (sacré bleu).
So what about deja-vu?
The phrase itself is made up of déjà meaning 'already' (note the accents that appear in the French version) and vu, the past participle of the verb voir, to see. It therefore means 'already seen'.
You'll hear the constituent parts all the time in daily conversation, as well as the words together in certain contexts - eg J'ai déjà vu ce film - I've already seen the film.
But the phrase déjà-vu, written with a hyphen, is also used in French, where it is masculine. The pronunciation is slightly different, leaning more on the second syllable of déjà - more like day-JHA - rather than on the first which is more common in English.
Le déjà-vu describes a sense of having already witnessed a scene or a conversation, accompanied by a slight feeling of strangeness or unreality.
It's a form of paramnesia, and is essentially caused by the wiring in your brain getting slightly mixed up and filing something as a memory rather than a new experience. It's estimated that around 70 percent of people experience it from time to time.
The reason that so many languages around the world use the French phrase is that the phenomenon was first described by the French philosopher Émile Boirac, in his 1876 book L’Avenir des sciences psychiques (the future of psychological science).
In everyday conversation, you can distinguish déjà-vu from a more normal sense of having already seen something by the pronoun - if it's le déjà-vu then it's referring to the spooky brain trick.
You may also hear une sensation du déjà-vu - a sense of deja-vu.
For example
Le déjà-vu peut être mineur, de faible durée, de quelques secondes à 1 ou 2 minutes, il apparaît et disparaît rapidement - Deja-vu can be minor, lasting from a few seconds to one or two minutes, appearing and disappearing rapidly
Je viens d'avoir un petit air de déjà-vu - I just had a bit of deja-vu
Je m'ennuie et j'ai déjà vu Netflix - I'm bored and I've already watched Netflix