Déjà vu - from 'already seen' in French - is the feeling that one has experienced, or even 'lived through' a particular situation before - but accompanied by the belief that, in reality, one hasn't.

It is an anomaly of memory whereby, despite the strong sense of recollection, the time, place, and practical context of the "previous" experience are uncertain or believed to be impossible.

Source Wikipedia

Déjà vu experiences are commonplace - around 60% of the population report they have experienced them at some point. Researchers have been trying to find explanations for more than a century.

The current mainstream theories include the :

• Split perception explanation
• Memory-based explanation
• Dream-based explanations

See Wikipedia

Note: Since the development of electronic brain-monitoring technology there have many more experimental investigations. Despite this, to date, there are still no widely accepted explanations.

Further in-depth info :Psychonomic Bulletin & Review, Volume 16, pages 1082–1088.

Note: Increased prevalence of Déjà vu is associated with Schizophreniaplugin-autotooltip__plain plugin-autotooltip_bigSchizophrenia

"Schizophrenia is a mental disorder characterized by abnormal social behavior and failure to understand what is real. Common symptoms include false beliefs, unclear or confused thinking, auditory and visual hallucinations, reduced social engagement and emotional expression, and a lack of motivation." and Epilepsyplugin-autotooltip__plain plugin-autotooltip_bigEpilepsy

"Epilepsy is a chronic noncommunicable disease of the brain that affects around 50 million people worldwide. It is characterized by recurrent seizures, which are brief episodes of involuntary movement that may involve a part of the body (partial) or the entire body (generalized) and are sometimes accompanied by loss of consciousness and control of bowel or bladder function. Seizure episodes are a result of excessive electrical discharges in a group of brain cells. Different parts of …