A demi-plane or demi-plane is a strange thing. It is similar to a plane with a few important features. Like normal planes, of course, it exists within the multiverse.
Demi-planes are of finite size unlike the infinite planes. Demi-planes usually, but not always, are found within existing planes. They usually have access through several methods and usually are connected to at least one other plane through a portal of some kind.
A lesser version of demi-planes are known as pocket dimensions. Demi-planes, in general, tend to be larger but that is not the only difference. Pocket dimensions tend to be created by mortals, whether through arcane, psionic, of divine means and in the eternal scheme of things tend to be transitory.
Demi-planes, on the other hand, require a great deal more power to create. Deities can create them, certainly, but many exist as strange folds or bubbles in the extant landscape of existing planes. Some argue that they are the expression of lesser known ideals, alignments, or elements beyond the basic four, their quasi-elemental cousins, the basic energy types and their para-elemental variants. Other wise sages know that they have been created in the past as private retreats and occasionally prisons for mighty creatures and deities so powerful and disruptive even the most powerful chaotic deities could not tolerate their unfettered existence.