Multiple personality disorder is a medical condition in which an individual exhibits two or more distinct personalities, often with separate memories and periods of control over the body.
In 2375, Seven of Nine experienced something akin to multiple personality disorder when a malfunctioning Borg vinculum tapped into her cortical implant and caused the neural patterns of assimilated individuals to emerge. The symptoms worsened until she began to show a different personality every few seconds, causing severe stress on her cerebral cortex and threatening her life. The crew of the USS Voyager was able to save her life by deactivating the vinculum. (VOY: "Infinite Regress")