a line or border separating two countries
边境，边界。
the district near such a line
边疆。
the extreme limit of settled land beyond which lies wilderness, especially referring to the western US before Pacific settlement
（靠近荒野的定居地的）边缘地带，边远地区，边疆（尤指美国人在太平洋沿岸定居之前的西部）
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his novel of the American frontier. 他写的关于美国西部边疆开拓的小说。
the extreme limit of understanding or achievement in a particular area
（某一学科）的知识（或成就）边缘，前沿，新领域
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the success of science in extending the frontiers of knowledge. 科学在开拓知识新领域方面所取得的成功。
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frontiern.边境；边界；边疆；边缘地带；边远地区；的知识（或成就）边缘；前沿；新领域
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frontn.前部；前面；正面；最前面部分；前线；前沿阵地；模样；举止；胆大；自信；脸；前额
"late Middle English： from Old French
