Definition of frontier:
(n) :
that part of a country which fronts or faces another country or an unsettled region; the marches; the border, confine, or extreme part of a country, bordering on another country; the border of the settled and cultivated part of a country; as, the frontier of civilization
"1940 "The frontier was not a line, but a district where thieving Scot and thieving Englishman had sufficient liberty for roving forays." W K Hancock. Survey of Commenwealth affairs Vol II part I p.3."
(adj) :
lying on the exterior part; bordering; conterminous; of or relating to a frontier
"a frontier town"