Dissociation is a psychological state or condition in which certain thoughts, emotions, sensations, or memories are separated from the rest of the psyche.
The French psychiatrist Pierre Janet (1859-1947) coined the term in his book L'Automatisme psychologique; he emphasized its role as a defensive manoeuver in response to psychological trauma. While he considered dissociation an initially effective defence mechanism that withdraws the individual psychologically from the impact of overwhelming traumatic events, a habitual tendency to dissociate would, however, promote psychopathology.
The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition considers symptoms such as depersonalization, derealization, and psychogenic amnesia to be core features of dissociative disorders. However, in the normal population mild dissociative experiences are highly prevalent, with 80% to 90% of the respondents indicating that they have had dissociative experiences at least some of the time. 
Attention to dissociation as a clinical feature is growing in recent years as a concommitant to knowledge of post-traumatic stress disorder, and as neuroimaging research and population studies show its relevance. Dissociation 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). Dissociation has a storied role in murder trials, or at least in movies about murder, where it is occasionally given as a reason for a "not guilty by reason of insanity" verdict.