split personality

1. A former name for "dissociative identity disorder," a psychiatric disorder in which a person maintains two or more seemingly separate, distinct, and enduring personality states. After she began having gaps in her memory, during which she displayed erratic speech and totally uncharacteristic behavior, her family began to suspect that she might have split personality.

2. A personality state characteristic of dissociative identity disorder. One of his split personalities is a fifty-year-old businessman from Scotland, a country he has never even visited.

3. A mood, mindset, mode of behavior, etc., that varies drastically from one's typical state. Man, it feels like the boss has split personalities sometimes. One day, he's your best friend, the next, he'll bite your head off! Janet has a real split personality that comes out on the weekends. She's so reserved and restrained during the week, but she's an absolute party animal whenever we go out to a club!