U

frontier

noun

the part of a country that borders another country; boundary; border.
the land or territory that forms the furthest extent of a country’s settled or inhabited regions.
Often frontiers.
1. the limit of knowledge or the most advanced achievement in a particular field: the frontiers of physics.
2. an outer limit in a field of endeavor, especially one in which the opportunities for research and development have not been exploited: the frontiers of space exploration.
Mathematics. boundary(def 2).

adjective

of, relating to, or located on the frontier: a frontier town.

noun

1. the region of a country bordering on another or a line, barrier, etc, marking such a boundary
2. (as modifier)a frontier post
US and Canadian
1. the edge of the settled area of a country
2. (as modifier)the frontier spirit
(often plural) the limit of knowledge in a particular fieldthe frontiers of physics have been pushed back

c.1400, from Old French frontier “prow of a ship, front rank of an army” (13c.), noun use of adjective frontier “facing, neighboring,” from front “brow” (see front (n.)).

Originally the front line of an army, sense of “borderland” is first attested early 15c. In reference to North America, from 1670s; later with 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]

Frontiersman is from 1782.