Lord Zack's Planescape
A demiplane is basically a small plane of existence. It is bounded unlike other planes that are (at least theoretically) infinite. They may, however span a very large area, at least up to the size of a material plane galaxy. For some of these planes it can be hard to know whether they are in fact demiplanes, or full-fledged planes.
Demiplanes are most commonly found in the Ethereal Plane. These can be similar to either the Material Plane or a Fundamental Plane, but often defy easy classification. There are also Spiritual demiplanes found in the Astral Plane, bits of belief that don’t fit anywhere else, kingdoms of spirits that have for one reason or another not made they’re way to the afterlife, including incorporeal undead, domains of deities that don’t wish to make they’re realms on a larger plane and others. Certain layers of planes may be demiplane-sized, but aren’t usually considered proper demiplanes due to they’re association with other planes.