1.(between countries)边疆；边界；边境
It wasn't difficult then to cross the frontier.当时过境并不难。
a far-flung outpost on the frontier遥远的边境前哨
2.(between people)界限
This is an easy way of crossing different social frontiers.这能轻松地跨越不同的社会界限。
3.<美>(edge)+of country边远地区；边区
4.+of science, technology尖端；最前沿
technological frontiers技术前沿
5.to push back the frontiers of sth
(break new ground)开拓…的新领域
We're pushing back the frontiers of science.我们在开拓新的科学领域。