Dissociative disorders are so-called because they are marked by dissociation from or an interruption of a person's fundamental aspects of waking consciousness (such as one's personal identity, one's personal history, etc.). Dissociative disorders come in many forms, the most famous of which is dissociative identity disorder (formerly known as multiple personality disorder). All of the dissociative disorders are thought to stem from trauma experienced by the individual with this disorder. The dissociative aspect is thought to be a coping mechanism. (American Psychiatric Association, 2000)
The person literally dissociates himself/herself from a situation or experience too traumatic to integrate with his conscious self. Symptoms of these disorders, or even one or more of the disorders themselves, are also seen in a number of other mental illnesses, including post-traumatic stress disorder, panic disorder, and obsessive compulsive disorder.