National boundary

Borders are geographic boundaries of political entities or legal jurisdictions, such as governments, sovereign states, federated states, and other subnational entities. Borders are established through agreements between political or social entities that control those areas; the creation of these agreements is called boundary delimitation.

Some borders—such as a state's internal administrative border, or inter-state borders within the Schengen Area—are often open and completely unguarded. Other borders are partially or fully controlled, and may be crossed legally only at designated border checkpoints and border zones may be controlled.

Borders may even foster the setting up of buffer zones. A difference has also been established in academic scholarship between border and frontier, the latter denoting a state of mind rather than state boundaries.

In the past, many borders were not clearly defined lines; instead there were often intervening areas often claimed and fought over by both sides, sometimes called marchlands. A special case in recent times was the neutral zones that were set up along parts of Saudi Arabia's borders with Kuwait and Iraq. (However, these zones no longer exist.). In modern times, marchlands have been replaced by clearly defined and demarcated borders. For the purposes of border control, airports and seaports are also classed as borders. Most countries have some form of border control to regulate or limit the movement of people, animals, and goods into and out of the country. Under international law, each country is generally permitted to legislate the conditions that have to be met in order to cross its borders, and to prevent people from crossing its borders in violation of those laws.

...
Wikipedia