A border is a defined line that separates two areas, often countries or regions. It can be marked by physical barriers like walls or fences, or simply by signs indicating the change in jurisdiction. Borders can be natural, such as rivers or mountains, or artificial, created through treaties or agreements. Borders play a crucial role in politics, trade, and security. They determine where one nation's laws apply and can affect immigration and customs. Disputes over borders can lead to conflicts, while well-defined borders can facilitate cooperation and economic exchange between neighboring areas, such as in the case of the European Union.