A closed interval corresponding to a finite portion of an infinite line. Line segments are generally labeled with two letters corresponding to their endpoints, say and , and then written . The length of the line segment is indicated with a vinculum, so the length of the line segment would be written .

Curiously, the number of points in a line segment is equal to that in an entire one-dimensional space (a line), and also to the number of points in an -dimensional space, as first recognized by Georg Cantor.