Plurality

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Plurality is the state of having multiple headmates collectively sharing a single body. A group of headmates is called a system. Plural experiences are extremely diverse; spiritual in nature and secular, median and partitonary, small systems or ones with thousands of headmates. There are many causes of plurality, including:
 * Traumagenic systems, resulting from a traumatic event or post-traumatic stress.
 * Systems created through tulpamancy or other explicit acts.
 * Other causes, which can be grouped under the umbrella term endogenic.
 * Mixed-origin systems, which have been created from a combination of causes.

Related Terms
singlet:

"Multiplicity" is a synonym of plurality, and are often used interchangeably. "Plural" is considered the umbrella term for all kinds of experiences as well — so much so that the wiki itself is named using it.

Singlet refers to someone who is not plural.