frontier (n.)

c. 1400, frowntere, "front line of an army;" early 15c., fronture, "borderland, part of a country which faces another," from Old French frontiere "boundary-line of a country," also "frontier fortress; front rank of an army" (13c.), noun use of adjective frontier "facing, neighboring," from front "brow" (see front (n.)). In reference to North America, "part of the country which is at the edge of its settled regions" from 1670s. Later it was given a specific sense:

What is the frontier? ... In the census reports it is treated as the margin of that settlement which has a density of two or more to the square mile. [F.J. Turner, "The Frontier in American History," 1920]