A border was a type of geographic boundary.

Kahler-Tek established a border surrounding the town of Mercy, Nevada. (TV: A Town Called Mercy)

The partition of India established a border between the nations of India and Pakistan. (TV: Demons of the Punjab)

UNIT sent a hypersonic jet past the border into Russia to enlist their resources in launching a warhead against the Cybermen. (TV: The Invasion)

In the 2010s, the Mexico border was among the places where newly-hatched Zygons were settled across the world as part of Operation Double, resulting in a minority of Zygons residing in New Mexico under the pretence of being British migrants. (TV: The Zygon Invasion)

In 2017, the armies of America, Russia and China converged near the border in Turmezistan, where the Monks' pyramid appeared. (TV: The Pyramid at the End of the World)

At the beginning of the New Dalek Paradigm, they established several "space fortresses", built along the borders of the Draconian, Sontaran and Morok Empires. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)