frontier

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An international border.
2. n. The area along an international border.
3. n. A region just beyond or at the edge of a settled area.
4. n. An undeveloped area or field for discovery or research: theories on the frontier of astrophysics.

Century Dictionary and Cyclopedia

1. n. That part of a country which fronts or faces another country; the confines or extreme part of a country bordering on another country; the marches; the border.
2. n. That part of a country which forms the border of its settled or inhabited regions: as (before the settlement of the Pacific coast), the western frontier of the United States.
3. n. A fort; a fortification.
4. n. The front or bordering part of anything, as the forehead.
5. n. Antagonistic or insolent bearing or aspect. [The sense of the word in the following passage is disputed.
6. n. Synonyms Border, Confine, etc. See boundary.
7. Of or pertaining to a frontier; lying on the border or exterior part; bordering: as, a frontier town.
8. Fronting; opposite.
9. To form or constitute a frontier; possess territories bordering on or constituting a frontier: with on or upon.
10. To place on the frontier; border.

Wiktionary

1. 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.
2. adj. Lying on the exterior part; bordering; conterminous.
3. adj. Of or relating to a frontier.

GNU Webster's 1913

1. 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.
2. n. An outwork.
3. adj. Lying on the exterior part; bordering; conterminous.
4. adj. Of or relating to a frontier.
5. v. To constitute or form a frontier; to have a frontier; -- with on.

WordNet 3.0

1. n. an undeveloped field of study; a topic inviting research and development
2. n. a wilderness at the edge of a settled area of a country
3. n. an international boundary or the area (often fortified) immediately inside the boundary