Definition [Cambridge Dictionary]: a real or imagined line that marks the edge or limit of something [边界；界线。多指作为界线的标识物等]

Example:

The Ural mountains mark the boundary between Europe and Asia.

乌拉尔山脉是欧洲和亚洲的分界线。

The fence marks the boundary between my land and hers.

这道栅栏是我的地和她的地的分界线。

Definition [Cambridge Dictionary]: a line that has been agreed to divide one country from another [边界，多指两国或两地之间的分界处附近的边缘地区、边界地带；也可以指物体的边缘等。]

Example:

Were you stopped at the border?

你在边界被拦住了吗？

We crossed the Mexican borders into the USA.

我们穿过墨西哥边界进入美国境内。

Definition [Cambridge Dictionary]: a border between two countries [边界，边境，边疆，指靠近边界的区域；也可引申为"未开发的领域"，"(学术的)前沿"等]

Example:

The American West was still a frontier a century ago.

一个世纪以前美国西部还是一片边疆。

She is a biochemist who works on the frontiers of biology.

她是从事生物学前沿研究的生化学家。