WordNet (r) 3.0 (2006):

heartland
    n 1: the central region of a country or continent; especially a
         region that is important to a country or to a culture

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "heartland":
   airspace, area, belt, confines, continental shelf, corridor,
   country, department, district, division, environs, ground,
   hinterland, inland, inlands, interior, land, midland, midlands,
   milieu, neighborhood, offshore rights, part, parts, place,
   precincts, premises, purlieus, quarter, region, salient, section,
   soil, space, terrain, territory, three-mile limit,
   twelve-mile limit, up-country, vicinage, vicinity, zone