...more common than you think

"Plurality (or multiplicity) is the existence of multiple self-aware entities inside one physical brain."

From More Than One, found below.

...That might sound crazy! It's certainly a very odd experience. Whether you're skeptical, or just curious, this section of this site explains the basics and links to other sites that go into more detail.

The explains some common terms.

External resources:

• Plurality Playbook, a resource for employers and coworkers, which covers pretty much everything the below sites do, I think.
• More Than One, an excellent starting point for anyone else.
• Plurality For Skeptics, for those who find the idea dubious.
• Rules of Engagement, a guide to plural etiquette.