From Pluralpedia, the collaborative plurality dictionary
realm (n., adj.)
Applies toheadspaces, system functions

Realms are specific areas in the headspace that usually have their own themes and are separate parts, sort of in an island-like fashion (but on a much bigger scale).

Realms will usually only be accessible by specific members for that realm. System members can also live on realms, or they can be used for things such as forming members, holding dormant ones, or keeping hostile members away from everyone else.