[C] (a) ~ (between sth and sth); ~ (with sth) border between two countries 国界; 边界: the frontier between Austria and Hungary 奥地利和匈牙利之间的边界. (b) land on either side of such a border 边疆; 边陲; 边境; 国境: [attrib 作定语] a frontier zone 边境地带 * a frontier town 边疆城镇 * frontier disputes 边界争端. 
the frontier [sing] (esp US) extreme limit of settled land, beyond which the country is wild and undeveloped 已开发地区的边缘地带: Beyond the frontier lay very real dangers. 在开发地区以外十分危险. 
the frontiers [pl] extreme limit, esp of knowledge about sth 极限（尤指有关某事物的知识的）; 尖端; 边缘: advance the frontiers of science 拓展科学的领域 * teach near the frontiers of one's subject, ie give recently discovered information 讲授个人专业领域的新知识. =>Usage at border 用法见border.> frontiersman / -zmEn; -zmEn/ n (pl -men / -mEn; -mEn/) man living on the frontier; one of the first settlers of an area 边疆居民; 边民; 拓荒者; 开拓者.