Dissociative Identity Disorder
One way to look at multiple personality is to look at how "professionals" in the field of psychology view the phenomenon. Currently, the phenomenon is called Dissociative Identity Disorder.
In the DSM-III and DSM-III-R, there was Multiple Personality Disorder (MPD). With the advent of the DSM-IV, the diagnosis changed to Dissociative Identity Disorder -- otherwise known as DID, and is now in the same general group of disorders as post-traumatic-stress-disorder (PTSD).
Many people who have multiple personalities find adding "disorder" to their state of being objectionable. Medicalization of something that is different from some unidentifiable "norm" is rampant in this day-and-age, and it is possible that multiples are expressing one end of a spectrum of behavior and self-structuring.
For educational purposes, the definition of Dissociative Identity Disorder is below:
Diagnostic criteria for 300.14 Dissociative Identity Disorder
A. 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).
B. At least two of these identities or personality states recurrently take control of the person's behavior.
C. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.
D. 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.
Reprinted without permission, but within the confines of fair use (negligable portion of an extensive text which does not compromise the integrity of the entire text, and well within the confines of educational purposes in a non-profit forum), from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR). Copyright 2000 American Psychiatric Association.