Déjà Vu or Deja Vu can be described as the sudden sensation that a current event or situation has already been experienced at some point in the past when, in actuality, it hasn't.     This term is a common phrase from the French language which translates literally into “already seen” and is directly related to the lesser known state referred to as "jamais-vu". It is a well-documented phenomenon that can commonly occur throughout both sober living and under the influence of hallucinogens.
Within the context of psychoactive substance usage, many compounds are commonly capable of inducing spontaneous and often prolonged states of mild to intense sensations of déjà vu. This can provide one with an overwhelming sense that they have “been here before”. The sensation is also often accompanied by a feeling of familiarity with the current location or setting, the current physical actions being performed, the situation as a whole, or the effects of the substance itself.
This effect is often triggered despite the fact that during the experience of it, the person can be rationally aware that the circumstances of the “previous” experience (when, where, and how the earlier experience occurred) are uncertain or believed to be impossible.
Déjà vu is often accompanied by other coinciding effects such as short-term memory suppression and thought loops. It is most commonly induced under the influence of moderate dosages of hallucinogenic compounds, such as psychedelics, dissociatives, and cannabinoids.
It is worth noting that the experience of Déjà vu is extremely common in people who are completely sober. For example, a 2003 review found that roughly two-thirds of the general population have had déjà vu experiences.  Other studies confirm that déjà vu is a common experience in healthy individuals, with between 60% and 70% of individuals reporting it, particularly within those who are between the ages of 15 and 25.  explanations of déjà vu typically conclude that this state of mind is as an anomaly of memory, which creates the distinct impression that an experience is "being recalled" when it is actually occurring within the present moment.   This explanation is supported by the fact that the sense of "recollection" at the time is strong in most cases, but the circumstances of the "previous" experience (when, where, and how the earlier experience occurred) are believed to be improbable or impossible.
Another plausible theory attributes the feeling of having previously experienced something that is currently experienced to that of having dreamt about a similar situation or place and then forgetting about it until one is mysteriously reminded of the situation or the place while awake. 
- Brown, A. S. (2003). A review of the deja vu experience. Psychological bulletin, 129(3), 394. | https://doi.org/10.1037/0033-2909.129.3.394
- O’Connor, A. R., & Moulin, C. J. (2010). Recognition without identification, erroneous familiarity, and déjà vu. Current Psychiatry Reports, 12(3), 165-173. | https://doi.org/10.1007/s11920-010-0119-5
- Deja Vu definition (AlleyDog) | https://www.alleydog.com/glossary/definition.php?term=Deja+Vu
- Deja Vu definition (Psychology Dictionary) | https://psychologydictionary.org/deja-vu/
- The Déjà Vu Illusion (SageJournals) | http://journals.sagepub.com/doi/10.1111/j.0963-7214.2004.00320.x
- The Neuroscience of Déjà Vu (PsychologyToday) | https://www.psychologytoday.com/blog/brain-babble/201208/the-neuroscience-d-j-vu