Realm

A royal jurisdiction or domain; a region which is under the dominion of a king; a kingdom.

Hence, in general, province; region; country; domain; department; division; as, the realm of fancy.

a knowledge domain that you are interested in or are communicating about; "it was a limited realm of discourse"; "here we enter the region of opinion"; "the realm of the occult"

a domain in which something is dominant; "the untroubled kingdom of reason"; "a land of make-believe"; "the rise of the realm of cotton in the south"