Definition.
Dissociationism is a forerunner of depth psychology dating primarily to the late-19th to mid-20th centuries, but reaching back as early as the 1700s. This school of thought suggests that the nature of our psyche is largely dissociative — that our functioning is guided by how we separate (rather than associate) ideas, memories, affects, etc.
As a school of thought, dissociationism was important in developing the practice we now call hypnosis. More, during hypnotic practices, dissociationists discovered that some individuals seemed to be a plurality of persons, having multiple identities. In this way, hypnotism and multiple personality were conceptually paired, and both became subjects of dedicated, often fairly scientific dissociationist study.
Following the work of Ernest R. Hilgard — who, in his book, Divided Consciousness (1986), proposed a neodissociationism — I advocate a new dissociationism, an interdisciplinary model which emphasizes psychic plurality through an appreciative and non-pathologizing lens.