the part of a country that borders another country; boundary; border.

the land or territory that forms the furthest extent of a country's settled or inhabited regions.

Often, frontiers.

the limit of knowledge or the most advanced achievement in a particular field

the frontiers of physics.

an outer limit in a field of endeavor, esp. one in which the opportunities for research and development have not been exploited

the frontiers of space exploration.

Mathematics . boundary ( def. 2 ) .

of, pertaining to, or located on the frontier

a frontier town.

noun

metropolis

noun

borderland , borderline , bound , confines , edge , limit , march , perimeter , verge , backcountry , backwater , backwoods , boondocks * , boonies , bush , hinterland , outback , outskirts , sticks , unknown * , boundary , marchland , border , outpost , unsettled