I. noun
1.
a region between two countries
lived on the frontier between Mexico and the U.S.
Synonyms: border, borderland, march, marchland
2.
a rural region that forms the margin of settled or developed territory
living on the frontier was a hard life
Synonyms: backcountry, backland, ||backveld, backwash, backwater, backwoods, ||boondocks, ||boonies, bush, hinterland, ||outback, sticks, up-country
Idioms: the back of beyond
II. adjective
Synonyms: back 1, outlandish, remote, unsettled