WordNet sense 1 (a particular environment or walk of life):

arena, domain, field, orbit, sphere

WordNet sense 2 (a particular geographical region of indefinite boundary (usually serving some special purpose or distinguished by its people or culture or geography)):

country

WordNet sense 3 (a part of an animal that has a special function or is supplied by a given artery or nerve):

region

WordNet sense 4 (the extent of a 2-dimensional surface enclosed within a boundary):

expanse, surface area