frontier UK /ˈfɹʌntɪə/US /fɹʌnˈtɪɹ/noun 3verb 2forms 3 noun3 •The part of a country which borders or faces another country or unsettled region.For two years the Easterner had been searching for his lost brother, at trading posts and Army forts all along the frontier. — 1950 •The most advanced or recent version of something; the leading edge. •(no longer used) An outwork of a fortification.thou hast talk'd. Of sallies and retires, of trenches, tents, Of palisadoes, frontiers, parapets — 1597 verb2 •To live as pioneers on frontier territory. •(no longer used) To place on the frontier.now that it is no more a Border, nor frontiered with Enemies, why should such Privileges be any more continued? — 1596 formsfrontiered · frontiering · frontiers