frontier Definition

1a. The part of a country bordering onto another country;
1b. A line, barrier, etc marking the boundary between two countries.
2. Limits.

Example: the frontiers of knowledge
Thesaurus: edge, boundary, verge, limit, borderline, confines, perimeter.
Form: frontiers
(N Amer)
3. historical
The furthest edge of civilization, habitation or cultivation, beyond which the country is wild and thought to be deserted.