<unk> is a state of acute mental decompensation in which certain thoughts, emotions, sensations, and/or memories are compartmentalized because they are too overwhelming for the conscious mind to integrate. This subconscious strategy for managing powerful negative emotions is sometimes referred to as "splitting", as these thoughts, emotions, sensations, and/or memories are "split off" from the integrated ego.  This use of the word "splitting" here should not be confused with references to splitting mentioned with regard to borderline personality disorder or family relations theory.
The French psychiatrist Pierre Janet(1859-1947) initially coined the term splitting in his book L'Automatisme psychologique.  There, he emphasized its role as a defensive mechanism employed in response to psychological trauma. While he considered<unk> an initially effective defense mechanism that protects the individual psychologically from the impact of overwhelming traumatic events, a habitual tendency to dissociate would likely be a marker of a more pronounced psychopathology.
The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition considers symptoms such as depersonalization (DSM-IV Codes 300.6), derealization, and Dissociative Amnesia (DSM-IV Codes 300.12) to be core features of dissociative disorders (DSM-IV Codes Dissociative Disorders). However, in the normal population mild dissociative experiences are highly prevalent, with 60% to 65% of the respondents indicating that they have had dissociative experiences at least some of the time. 
Attention to<unk> as a clinical feature has been growing in recent years as a concomitant to knowledge of post-traumatic stress disorder, and as neuroimaging research and population studies show its relevance.<unk> most often makes the news with regards to soldiers' responses to wartime stress, rape victims with amnesia for details, and in occasional criminal trials where the question of whether a person with Dissociative Identity Disorder (DID) can be responsible for his or her actions.
Perhaps the most widely-known form of dissociative disorder is DID, formerly known as Multiple Personality Disorder (MPD)
(see Multiple personality controversy).
Psychoactive substances can often induce a state of temporary<unk>  Substances with dissociative properties include ketamine, nitrous oxide, tiletamine, DXM and PCP.
Dissociation 
Withdrawal
Exaptation
Attachment 
Supination
Impaction
Perseveration
Derealization