Déjà vu Phenomenon
Déjà vu is a French phrase and literally means “already seen”. Those who have experienced the feeling describe it as an overwhelming sense of familiarity with something that shouldn't be familiar at all. Term variations of deja vu, include deja vecu, meaning "already experienced", or deja senti, meaning "already felt".
This phenomenon also called paramnesia, from Greek παρα "para," "near" + μνήμη "mnēmē," "memory" or promnesia, is the experience of feeling sure that one has witnessed or experienced a new situation previously (an individual feels as though an event has already happened or has happened in the near past), although the exact circumstances of the previous encounter are uncertain.
The term was coined in 1876 by French scientists Emile Boirac in his book called "L'Avenir des Sciences Psychiques". Although he was among the first to study this bizarre phenomenon, about which we know so little and for no small reason, since it involves the complex human mind, he didn't make in depth investigations.
In Layman terms, Deja vu is, a human perception about something that was recently experienced or seen before the person involved knows he/she hasn't. A common misinterpretation of the term is related to precognitive experiences or dreams, where the person knows exactly what is going to happen and it turns out that he/she is right.
This theory was supported by many, starting from Swiss scientist Arthur Funkhouser, then by aeronautical engineer J.W. Dunne who actually conducted studies on this matter in 1939, and Nancy Sondow, who also ran a study in 1988 with almost identical results. Nevertheless, the distinction between the two is that while deja-vu's are experienced DURING an event, precognitive experiences occur BEFORE something actually happens.
The experience of déjà vu seems to be quite common among adults and children alike; in formal studies, 70% of people report having experienced it at least once. References to the experience of déjà vu are also found in literature of the past, indicating it is not a new phenomenon. It has been extremely difficult to evoke the déjà vu experience in laboratory settings, therefore making it a subject of few empirical studies. Recently, researchers have found ways to recreate this sensation using hypnosis.