When someone has multiple personalities that they switch between at different times. Characterized by each personality not having full accesss to the memories of the others. Also, each personality often has totally different mannerisms, penmanship, and even voice patterns.

Often popularily called schizophrenia, though that disorder has nothing to do with this one.

The first documented case of multiple-personality disorder, as it was called at the time, is the movie "The Three Faces of Eve." It portrayed one of the first known cases of multiple-personality in America: a lady who had three different personalities. However, since that time, the name of the disorder has been changed to Dissociative Identity Disorder. For that reason, more scientific and a greater quantity of information can be found at that node.