An international border is a defined line that separates two countries. It marks the legal boundary where one nation's laws and regulations end, and another's begin. These borders can be established through treaties, historical claims, or negotiations and can vary in length and complexity. Borders can be physical, like rivers or mountains, or they can be invisible lines on a map. They play a crucial role in international relations, trade, and security. Countries often have customs and immigration controls at these borders to manage the movement of people and goods, ensuring compliance with their respective laws.