Dissociative Identity Disorder (DID) is defined (in the US DSM-5) as "the presence of two or more distinct personality states".

It was previously known as Multiple Personality Disorder, or colloquially as Split Personality.

It's often associated - in an unexplained way - with highly traumatic experiences in a patient's past.

The prevalence has been estimated to be as high as 1.5% of the general population.

Despite research on DID including structural and functional magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, event-related potential, and electroencephalography, no convergent neuroimaging findings have been identified regarding DID, making it difficult to hypothesize a biological basis for DID.
Source : Wikipedia