1. noun
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.
2. adjective
a) Lying on the exterior part; bordering; conterminous.
a frontier town
b) Of or relating to a frontier.
Wikipedia foundation.