Frontier, n.
Etymology: [F. frontière, LL. frontaria. See Front.]
1. That part of a country which fronts or faces another country or an unsettled region; the marches; the border, confine, or extreme part of a country, bordering on another country; the border of the settled and cultivated part of a country; as, the frontier of civilization.
2. (Fort.)
An outwork. [Obs.] Palisadoes, frontiers, parapets. Shak.
2 other Webster entries under this spelling
Frontier, a.
1. Lying on the exterior part; bordering; conterminous; as, a frontier town.
2. Of or relating to a frontier. "Frontier experience." W. Irving.
Frontier, v. i.
To constitute or form a frontier; to have a frontier; -- with on. [Obs.] Sir W. Temple.
Shades of meaning
Fernald has no entry of its own for frontier; he lists it under boundary.
He lists it without comment; the entries above explain its neighbors.