Definition of Frontier
In international law, that portion of the territory of any country which lies close along the border line of another country, and so "fronts” or faces it. Border between two countries. The term means something more than the boundary line itself, and includes a tract or strip of country, of indefinite extent, contiguous to the line.
That's the definition of Frontier in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.