the part or edge of a surface or area that forms its outer boundary.

the line that separates one country, state, province, etc., from another; frontier line: You cannot cross the border without a visa.

the district or region that lies along the boundary line of another.

the frontier of civilization.

the border. the border between the U.S. and Mexico, especially along the Rio Grande. (in the British Isles) the region along the boundary between England and Scotland.