the part of a country that borders another country; boundary; border.
the land or territory that forms the furthest extent of a country's settled or inhabited regions.
Often, frontiers. the limit of knowledge or the most advanced achievement in a particular field: the frontiers of physics. an outer limit in a field of endeavor, especially one in which the opportunities for research and development have not been exploited: the frontiers of space exploration.
Mathematics, boundary (def 2).
of, relating to, or located on the frontier: a frontier town.