A demarcation is a line, boundary, or other conceptual separation between things. Geographically, a demarcation might be the border that separates two countries or the river that divides two regions.
Demarcation comes from the German word for mark. Although it's often used as a term regarding geographical borders and boundaries, it can also be used to describe lines between categories or groups. For example, a definite demarcation exists between people who love the Boston Red Sox and those who love the New York Yankees.