frontier

1. N-COUNT When you are talking about the western part of America before the twentieth century, you use frontier to refer to the area beyond the part settled by Europeans. (20世紀(jì)前美國) 西部邊疆

...a far-flung outpost on the frontier. …遠(yuǎn)駐西部邊疆的一個哨所。

2. N-COUNT Thefrontiers of something, especially knowledge, are the limits to which it extends. (尤指知識的) 前沿

...pushing back the frontiers of science. …推進(jìn)科學(xué)前沿。

3. N-COUNT A frontier is a border between two countries. 國界

It wasn't difficult then to cross the frontier. 那時候穿越國界并不難。