Dissociative Identity Disorder aka Multiple Personality Disorder

The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self).

At least two of these identities or personality states recurrently take control of the person's behavior. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness. The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during Alcohol Intoxication) or a general medical condition (e.g., complex partial seizures). Note: In children, the symptoms are not attributable to imaginary playmates or other fantasy play.

Dissociative Identity Disorder is also referred to as Multiple Personality Disorder.