Derealization or derealisation (sometimes abbreviated as DR) is the experience of feeling detached from, and as if one is an outside observer of, one's surroundings (e.g., individuals or objects are experienced as unreal, dreamlike, foggy, lifeless, or visually distorted).  It's a type of cognitive and perceptual dysregulation.  People experiencing derealization often claim that reality persistently feels as if it is a dream, or something watched through a screen,  like a film or video game. These feelings can sometimes instill the person with a sensation of alienation and distance from those around them.
Derealization is not an inherently negative altered state of awareness, as it does not directly affect one's emotions or thought patterns. However, derealization can sometimes be distressing to the user, who may become disoriented by the loss of the innate sense that their external environment is genuinely real. This loss of the sense that the external world is real can in some cases make interacting with it feel inherently inauthentic and pointless.
This state of mind is commonly associated with and often coincides with the very similar psychological state known as depersonalization. While derealization is a perception of the unreality of the outside world, depersonalization is a subjective experience of unreality in one's sense of self.
Derealization is often accompanied by other coinciding effects such as anxiety and depersonalization.  It is most commonly induced under the influence of moderate dosages of dissociative compounds, such as ketamine, PCP, and DXM. However, it can also occur to a less consistent extent with any anxiety ridden drug experience or during the withdrawal symptoms of stimulants and depressants.
In psychology, chronic derealization that persists during sobriety for prolonged periods of time, is not attributable to another disorder and is distressful to the person, is identified as "derealization disorder", classified by the DSM-IV as a dissociative disorder.
While degrees of derealization are common and can happen temporarily to anyone who is subject to an anxiety or stress-provoking situation, chronic derealization is more common within individuals who have experienced a severe trauma or prolonged stress and anxiety. The symptoms of both chronic derealization and depersonalization are common within the general population, with a lifetime prevalence of up to 26-74% and 31–66% at the time of a traumatic event. 
It has been demonstrated that derealization may be caused by a dysfunction within the brains visual processing centre (occipital lobe) or the temporal lobe, which is used for processing the meaning of sensory input, language comprehension, and emotion association. 
- American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.), 818-20. Arlington, VA: American Psychiatric Publishing. | https://doi.org/10.1176/appi.books.9780890425596.GlossaryofTechnicalTerms
- Espiard, M. L., Lecardeur, L., Abadie, P., Halbecq, I., & Dollfus, S. (2005). Hallucinogen persisting perception disorder after psilocybin consumption: a case study. European Psychiatry, 20(5), 458-460. | https://doi.org/10.1016/j.eurpsy.2005.04.008
- Hunter, E. C., Sierra, M., & David, A. S. (2004). The epidemiology of depersonalisation and derealisation. Social psychiatry and psychiatric epidemiology, 39(1), 9-18. | https://dx.doi.org/10.1007/s00127-004-0701-4
- Sierra, M., Lopera, F., Lambert, M. V., Phillips, M. L., & David, A. S. (2002). Separating depersonalisation and derealisation: the relevance of the “lesion method”. Journal of Neurology, Neurosurgery & Psychiatry, 72(4), 530-532. | http://dx.doi.org/10.1136/jnnp.72.4.530