Plurality

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 partionary, 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
"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.

Credits
There's no one coiner for the term "plural" in this context. However, many resources can be found using such wording, including
 * https://morethanone.info/
 * http://healthymultiplicity.com/#i
 * https://powertotheplurals.com/