This page tells you what a border is.

Here is a definition of "border" from

"The line that separates one country, state, province, etc. from another, frontier line; You
cannot cross the border without a visa."

This website talks specifically about the US/Mexico border which in some places is marked

with a fence or border wall but in other places is simply an imaginary line. To learn more

about the US Mexico border, visit the History of the Border page.

The Border Wall

The US/Mexico Border