Definition of "frontier"

From time to time the coaches of the Lötschberg Railway itself, which in comfort and décor can rank with the finest in Europe today, travel far from the frontiers of Switzerland on through workings such as these.

1960 December, Cecil J. Allen, “Operating a mountain main line: the Bern-Lötschberg-Simplon: Part One”, in Trains Illustrated, page 743

Unlike a boundary, which evokes the image of a line on a map and demarcates spheres of political control, the frontier is an area where colonisation is taking place....no authority is recognised as legitimate by all parties or is able to excersise undisputed control over the area.

1979, Richard Elphic, Hermann Guilomee (editors), The shaping of South African Society, 1652 - 1820, page 297

thou hast talk'd. Of sallies and retires, of trenches, tents, Of palisadoes, frontiers, parapets

c. 1597 (date written), William Shakespeare, “The First Part of Henry the Fourth, […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act II, scene iii]

now that it is no more a Border, nor frontiered with Enemies, why should such Privileges be any more continued?

1596 (date written; published 1633), Edmund Spenser, A Vewe of the Present State of Irelande […], Dublin: […] Societie of Stationers, […]; republished as A View of the State of Ireland […] (Ancient Irish Histories), Dublin: […] Society of Stationers, […] Hibernia Press, […] [b]y John Morrison, 1809,