Simplify the following text for a Grade 13 native English-speaking person.
Original: The diagnosis of Dissociative identity disorder is defined by criteria in the American Psychiatric Association 's Diagnostic and Statistical Manual of Mental Disorders ( DSM ) . The DSM-II used the term multiple personality disorder , the DSM-III grouped the diagnosis with the other four major dissociative disorders , and the DSM-IV-TR categorizes it as dissociative identity disorder .
Simplified: Dissociative Identity Disorder ( DID ) is a medical diagnosis of a mental condition .