border

A border is a geographic line that separates political or administrative entities, such as nations, states, or municipalities. Borders may be natural, defined by rivers, mountain ranges, or coastlines; or artificial, created by surveying and treaties. They establish sovereignty, jurisdiction, and the legal framework for cross-border movement and trade.

International borders regulate who may enter and leave, and under what conditions. Border control agencies, customs, and immigration laws oversee crossings at ports of entry, airports, and land checkpoints. Some borders are open or eased, as with regional agreements; others are fortified or fenced for security, immigration control, or combating smuggling. Border disputes arise from competing territorial claims, shifting geographies, or historical legacies.

Historically, borders have shifted through treaties, wars, independence movements, and decolonization. Examples include the shifting borders of Europe after the World Wars, the border between India and Pakistan, and the U.S.–Mexico boundary. In the European Union, internal borders have been reduced or eliminated under the Schengen Area, while external borders remain controlled.

Other uses of the term border include in mathematics, where the boundary or edge of a set is the collection of points that can be approached both from inside and outside the set; in textiles or graphic design, a border decorates edges; and in computing, a border is an outline around a user interface element.