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).

Related Terms
An alter is the medical term for a headmate; they have interchangeable definitions, but it is up to a specific system to decide whatever terminology they prefer.

"System member" and "pluran" are other common ways to refer to headmates.