A realm is what you call the largest manner of space-magic, and can usually be characterized by being enveloped in an horizon sphere and containing many kinds of halls within it. It's usually so large you can't see the beginning or end while standing against one wall, and are by tradition usually circular.
Realms are considered a great feat of magic, and the space Magus capable of creating such is legendary. Usually, it takes many magi's mutual effort to spell one into being, and this is an historical event.
The Broken Tower is notorious for containing an almost obscene amount of realms.