Frontier

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.