(~s)
1.
A ~ is a border between two countries. (BRIT; in AM, usually use border )
It wasn’t difficult then to cross the ~.
N-COUNT
2.
When you are talking about the western part of America before the twentieth century, you use ~ to refer to the area beyond the part settled by Europeans.
...a far-flung outpost on the ~.
N-COUNT
3.
The ~s of something, especially knowledge, are the limits to which it extends.
...pushing back the ~s of science.
...technological ~s.
N-COUNT: usu pl, usu N of n, adj N