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, the frontiers of physics.
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, esp. one in which the opportunities for research and development have not been exploited:the frontiers of space exploration.
boundary (def. 2).
—adj.
of, pertaining to, or located on the frontier: a frontier town.