“Alter” is a term often given to the *alter*nate personality states experienced in DID and OSDD-1. These can vary in distinction from indistinct fragments to full-blown distinct identities.
Historically, alters have been referred to as multiple personalities, it’s now generally considered that they are instead dissociative parts of a single personality and are seen as divided psycho-biological systems.
Other terms that have been used to refer to these dissociative parts include (dissociative) self-states, ego states, (dissociative) identity states, (dissociative) selves, (dissociative) identities, (dissociative) parts, and (dissociative) personality states.
The online dissociative community has also adopted the term “headmates” to refer to these states in addition to those listed above.