A demiplane is a Plane of finite extent, often created by a spellcaster.
In Planescape, all demiplanes form from the protomatter of the Ethereal Plane.
There are a great number of these planes in existence, varying in form and shape. Some are as small as a house, someone nearly infinite in size. The Shadow Plane was once a demiplane, but is now categorized as a full-blown Transitive Plane.
Sources:
Dungeon Masters Guide
Planescape Campaign Setting
Manual of the Planes
Guide to the Ethereal Plane
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