Demiplane

Demiplanes are ill-defined planes of unusually small size that lie either within other planes, such as astral dominions, or outside of either the parallel planes or the fundamental planes. There are undoubtedly several of them, but most are unknown. Aside from those that lay within the dominions of the gods the best known is Sigil, also called the City of Doors, which acts as a strange interplanar nexus with hidden doors to every known plane—and many that remain largely unknown.

Demiplanes are typically, if not always, of finite size.