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Border (noun):
A border is an imaginary line or fence that separates cities, states, and countries all around the world. Our page is about the border between the United States and Mexico. This is an important border because it is a border that keeps people out of the United States, and also separates families.

"Many people risk everything to cross the border in search of a better life."

Also see: http://dictionary.reference.com/browse/border