A Realm is any location in the Umbra that has a consistent identity and form. Some Realms are small and temporary, such as the Paradox Realms that may swallow a mage to isolate them from the Consensus, while other Realms such as the Spirit Wilds are old and well-defined. The Technocratic Union refers to most Realms as Constructs.
Each Realm is metaphyiscally bounded by a pericarp, the local equivalent of the Gauntlet. Beyond that, however, Realms vary dramatically in their size and nature. Some Realms are inhabited only by ghosts, or spirits, or weirder things; others have a local Consensus tailored to the Paradigm of a particular school of magic, by coincidence or design. Small, personal realms anchored to real-world locations allow for a house, a car or a telephone booth to be "bigger on the inside than on the outside," and elder mages have been known to retire permanently to a custom-built pocket realm. Other realms, especially in the High Umbra, occur "naturally" in a dialectic relationship with the thoughts, dreams and ideas of human civilizations. The Void Engineers of the Technocracy and the Sons of Ether of the Traditions specialize in discovering and exploring new Realms.
The Avatar Storm destroyed or damaged many Realms, or broke their connections with Earth. Some of the mages trapped in those Realms underwent "Void Adaptation," losing their physical form to become spirit-like beings of pure thought. Mages attempting to travel to other Realms also found themselves stumbling into Hive Realms of unknown origin and purpose.