The term used for a line that divides two countries is called a "border." Borders can be natural, such as rivers or mountain ranges, or artificial, established through treaties, agreements, or historical events. They serve to delineate sovereignty and jurisdiction, impacting governance, trade, and cultural exchanges between nations. Borders can also represent varying levels of security, as some regions have heavily fortified boundaries while others maintain open borders. The dynamics of these borderlines can change over time due to politics, conflict, or diplomacy, shaping relationships between neighboring countries and influencing the lives of people living near them.