Paracosm

A Paracosm is a detailed imaginary world. To quote wikipedia: "Paracosms are thought generally to originate in childhood and to have one or numerous creators. The creator of a paracosm has a complex and deeply felt relationship with this subjective universe, which may incorporate real-world or imaginary characters and conventions. Commonly having its own geography, history, and language, it is an experience that is often developed during childhood and continues over a long period of time, months or even years, as a sophisticated reality that can last into adulthood."

While the wikipedia article does the topic justice, Paracosm are of particular interest to this site due to their occasional overlap with plurality. The intense personalities and character can become self-aware and thus headmates of parative origin.

Related Terms
Dreamway System are a specific type of Gateway System where one visits a paracosm.

Maladaptive Daydreaming Disorder, neuronarration, and Immersive Daydreaming all involve paracosm.