A FRONTIER is a border separating two countries. It is a physical or political boundary that marks the end of one country and the beginning of another. Frontiers are often marked by natural features such as rivers, mountains, or deserts, but can also be man-made, such as walls or fences. Historically, frontiers have been used to protect countries from invasion or to control immigration. In modern times, frontiers are used to facilitate trade and travel between countries.