Demiplane
A Demiplane is a term introduced in the Planescape setting for Dungeons & Dragons. In a nutshell, it refers to a "miniature" plane, one featuring well-defined geographical limitations as opposed to extending on to infinity in all directions. Demiplanes can be big - Sigil is technically a demiplane - but are usually associated with small sizes, from "big as a large room" to "the size of a mansion".
Creating demiplanes is a popular past-time for high level wizards, as it essentially lets them create their own personalized pocket of reality. With enough time and investment, a demiplane can grow into a full-fledged world in its own right.