Definition of "frontier" []

• An international border. (noun)
• The area along an international border. (noun)
• A region just beyond or at the edge of a settled area. (noun)
• An undeveloped area or field for discovery or research: theories on the frontier of astrophysics. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Serbia and Montenegro_: In the north of the Adriatic, _the entire coast, commencing from the Bay of Volosca_ on the frontier of Istria as far as the _northern frontier_ of Dalmatia, including the whole of the coast-line now belonging to Hungary, the entire coast of Croatia, the port of Fiume and the small harbours of Novi and Carlopago, as also the islands of Velia, Pervicchio, Gregorio, Goli and Arbe."
• "About two miles from the German frontier (_frontier_ just here as well as front) an isolated hill rises out of the Lorraine meadows."
• "The IMF groups 11 sub-Saharan economies into what it calls "frontier markets" that have natural resources, viable financial markets, and relative political stability."