Headmate

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A headmate is a singular person or entity in a plural system or collective. They can be clearly separated (like in DID or OSDD-1b, called a multiple or partitionary system) or more blurry and fluid (like in median or OSDD-1a). According to Astraea's Web, the term "headmate" originates from the soulbonding community, where it was used in systems without fictive soulbonds.

Related Terms
An alter or part is the medical term for a headmate, however it has never exclusively referred to systems with traumatic origins and these terms have interchangeable definitions. It is up to a specific system to decide whatever terminology they prefer. Personality is another medical term which has fallen out of use.

Median systems may have facets or aspects, rather than headmates.

"System member" and "pluran" are other common ways to refer to headmates. "Sysmate" is also used by some, particularly by those who dislike the connotation that system members are "in the head" or an exclusively mental phenomenon.