Literally "already seen": French gives it two jobs
Déjà means "already" and vu is the past participle of voir, "to see," so déjà vu is, word for word, "already seen." That much lines up with how English speakers use the phrase. French dictionaries distinguish a memory illusion from an everyday judgment of unoriginality. English dictionaries also document more than one sense. The Larousse dictionary lists the phrase as covering both "a memory disorder giving the sudden and intense impression of having already lived through the present situation in the past," and, separately, "something that is not new, that is banal, without originality."
That second sense appears in everyday French: c'est du déjà-vu is an ordinary way to say a film, a plot twist, or a political promise is predictable and unoriginal, with no memory illusion involved, just a shrug at something recycled.
The memory illusion and the figurative English use
Merriam-Webster defines déjà vu, in its main English sense, as "the illusion of remembering scenes and events when experienced for the first time": a feeling that you have seen or heard something before, even though you haven't. Alongside that memory-illusion definition, Merriam-Webster gives a second English sense for something overly or unpleasantly familiar, such as a poor sports season repeating a familiar pattern.
The term itself is relatively young as coined vocabulary: the French philosopher Émile Boirac first used it in 1876, in an open letter to the journal Revue Philosophique describing "le sentiment du déjà vu" as a specific type of memory illusion. He returned to the term later in his own writing on psychical research, but the coining moment is the 1876 letter, not a book. Either way, the psychological sense is a fairly recent, deliberately coined piece of vocabulary, not an old expression that gradually picked up its clinical meaning over centuries.
What's actually happening when you get it
Déjà vu is studied in memory research. Proposed explanations include a mismatch between familiarity and recollection, or partial resemblance to an earlier experience. The hippocampus is part of the medial temporal lobe and participates in memory; separating it from a supposed temporal-lobe memory store does not accurately describe that anatomy. A 2018 imaging study examined hippocampal subfields, but found no statistically significant volume difference between its healthy groups with and without déjà vu. It did not establish a timing glitch as the cause of an ordinary episode.
An isolated feeling of familiarity need not imply a neurological disorder. Clinical advice nevertheless depends on the pattern and accompanying symptoms. Repeated or unusually long experiences, or an episode accompanied by impaired awareness or other neurological symptoms, warrant a discussion with a clinician. A definition of the phrase cannot determine the cause of an individual episode.
The word for the opposite feeling
If déjà vu is something new feeling falsely familiar, jamais vu (literally "never seen") is the reverse: something genuinely familiar suddenly feeling strange or unrecognized. Cleveland Clinic describes it as the feeling that something you know well seems unreal or unfamiliar, and gives the common example of staring at an ordinary word until it stops looking like a real word. It's the less famous sibling of déjà vu, but it's a real, separately studied phenomenon, not just wordplay on the more famous term.
Déjà vu is one of the expressions English borrowed in its original form. Schadenfreude is another, from German.