From Middle English frounter, from Old French fronter (whence Modern French frontière), from front.

• (Received Pronunciation) IPA^(key): /ˈfɹʌntɪə/, /ˈfɹɒntɪə/, /ˈfɹʌntjə/, /ˈfɹɒntjə/, /fɹʌnˈtɪə/, /fɹɒnˈtɪə/^[1]
• (General American) IPA^(key): /fɹʌnˈtɪɹ/, /fɹɑnˈtɪɹ/^[2]
• IPA^(key): (obsolete) /ˈfɹʌnt͡ʃə/^[3]
• Rhymes: -ɪə(ɹ)
• Hyphenation: fron‧tier

• Audio (US):

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frontier (plural frontiers)

1. The part of a country which borders or faces another country or unsettled region.

   Synonyms: march, marches, border, marchland, borderland

   the frontier of civilization

   • 1955 April, “Notes and News: A Journey through Thailand”, in Railway Magazine, page 286:

     There were not many passengers, and the journey was uneventful until the frontier was reached at Padang Besar, although an armed escort of Malay police accompanied the train from Alor Star.

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

     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.

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

     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 [exercise] undisputed control over the area.

   • 2002 November 22, Steven Eke, “Russia's rocky relationship with Nato”, in BBC World Service‎^[2]:

     But there is also real opposition to Nato expansion - and even to Nato's continued existence after the Cold War.
     This is a view shared by a very large part of Russia's military and foreign affairs establishment.
     Educated and trained during the Soviet period, they are still inculcated with notions of super-power military might.
     It has been painful for these people to see Nato's outer edge creep ever closer to what was once the frontier of the USSR.

   • 2025 September 4, “What the splinternet means for big tech: Unpleasant new trade-offs, for starters”, in The Economist‎^[3]:

     The demise of “de minimis”, as the exemption is known, is the latest, symbolic reminder to digital darlings that frontiers exist—and that business models built on their absence are in need of a rethink.

2. The most advanced or recent version of something; the leading edge.

   Near-synonyms: vanguard, state of the art

   the frontier of solid state electronics

3. (obsolete) An outwork of a fortification.
   • 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, →OCLC, [Act II, scene iii]:

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

frontier (third-person singular simple present frontiers, present participle frontiering, simple past and past participle frontiered)

1. (intransitive) To live as pioneers on frontier territory.
2. (transitive, obsolete) To place on the frontier.
   • 1596 (date written; published 1633), Edmund Spenser, A Vewe of the Present State of Irelande[…], Dublin: […] Societie of Stationers,[…], →OCLC; republished as A View of the State of Ireland[…] (Ancient Irish Histories), Dublin: […] Society of Stationers, […] Hibernia Press, […] [b]y John Morrison, 1809, →OCLC:

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

• “frontier”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.