C2 [ C ] a border between two countries国境,边境
[ S ] the western US where settlers began to move to live in the 19th century(十九世纪时定居者开始搬入并居住的美国西部)前沿地带
More examples
The frontier is patrolled by border guards.
At the frontier, there were people pestering tourists for cigarettes, food or alcohol.
Two armed men were caught trying to cross the frontier at night.
Air pollution is no respecter of national frontiers.
The president pledged to respect the existing frontiers between the two countries.
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