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.-an outwork.-lying on the exterior part; bordering; conterminous; as, a frontier town.-of or relating to a frontier.-to constitute or form a frontier; to have a frontier; -- with on.