demesne,
domain,
land (mathematics) the set of values of the independent variable for which a function is defined
realm -
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"
realm -
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"