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世纪前美国) 西部边疆

...a far-flung outpost on the frontier. …远驻西部边疆的一个哨所。

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

...pushing back the frontiers of science. …推进科学前沿。

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

It wasn't difficult then to cross the frontier. 那时候穿越国界并不难。