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)