frontier
n.-
1 a border (n.) B2 Upper Intermediatethe line or area where one country touches another.
the political and geographical boundary separating two countries or territories.
ExampleThe soldiers were sent to guard the mountain frontier between the two nations.
ExampleTensions escalated as both nations increased their military presence along the disputed frontier, leading to a complete halt in cross-border trade.
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2 limit of knowledge (n.) C1 Advanced Academicthe furthest point of what is known in a particular subject or activity.
the extreme limit of understanding or achievement in a particular field.
ExampleSpace is often called the final frontier for human exploration.
ExampleThe latest research in quantum computing represents the new frontier of modern physics, challenging our fundamental understanding of information.
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3 unsettled land (n.) B2 Upper Intermediate American Englisha region at the edge of a settled area, especially in historical North America.
a region that forms the margin of settled or developed territory, often associated with historical expansion.
ExampleLife on the American frontier was difficult and dangerous for early settlers.
ExampleThe myth of the frontier has deeply influenced American literature, portraying the wilderness as a place of both lawless danger and infinite opportunity.
UsageOften used with the definite article 'the'.
From Middle English frounter, from Old French fronter (whence Modern French frontière), from front.