Dissociative identity disorder is a psychiatric diagnosis that describes a condition in which a person displays multiple distinct identities or personalities (known as alter egos or alters), each with its own pattern of perceiving and interacting with the environment.
In the International Statistical Classification of Diseases and Related Health Problems the name for this diagnosis is multiple personality disorder. In both systems of terminology, the diagnosis requires that at least two personalities routinely take control of the individual's behavior with an associated memory loss that goes beyond normal forgetfulness; in addition, symptoms cannot be the temporary effects of drug use or a general medical condition. DID is less common than other dissociative disorders with an incidence of approximately 1%, and is often comorbid with other disorders.
Miley Cyrus / Hannah Montana
Find more at:
 - http://en.wikipedia.org/wiki/Dissociative_identity_disorder