The border between two countries or regions is the dividing line between them. Sometimes the border also refers to the land close to this line.
They fled across the border.
...the isolated jungle area near the Panamanian border...
A country that borders another country, a sea, or a river is next to it.
...the European and Arab countries bordering the Mediterranean.
A border is a strip or band around the edge of something.
...pillowcases trimmed with a hand-crocheted border.
In a garden, a border is a long strip of ground along the edge planted with flowers.
...a lawn flanked by wide herbaceous borders.
If something is bordered by another thing, the other thing forms a line along the edge of it.
... the mile of white sand beach bordered by palm trees and tropical flowers
Caesar marched north into the forests that border the Danube River.