Alter

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Alter is often a synonym for headmate, but it's also used (especially in psychiatric contexts) to mean system members who are not the core.

History
Alter is usually a term used by traumagenic systems, such as those with DID, OSDD-1, UDD/USDD, and DDNOS. Non-traumagenic systems may use it, but it's more commonly used in these areas.

While the term alter is not mentioned in the DSM-5, it does contain the criterion of multiple personality states being present in one with dissociative identity disorder (DID DSM-5 criteria )

The term "alter" has been used to refer to system members since at least 1982 and may have been proposed as the preferred term for system members (in contrast to "personality") by Coons in this paper in 1984, according to Kluft. The descriptors "altered state of consciousness", "alternative states of consciousness", and "alter personalities" may have been used as early as 1889 by Pierre Janet, though they may have been applied to reference his work only later by van der Hart. Since the original publication is in french, pinpointing a connection to the English term is difficult. Janet describes "states" and often refers to "alternations" in behaviour and memory. Therefore, the term "alter" could be derived from either "alternate" or "altered". The terms "alter self" and "alter personality" have been used since at least 1972.

Related Terms
Headmate, pluran, sysmate, and system member are all words used with the same meaning as alter, without the psychiatric contexts.

Alters can have different physical affects, accents, memories, ages, names, functions, genders and other traits. The alter who has executive control over the body most of the time is known as the host. The host personality may or may not be the core.