The extension of a line segment refers to the extension of a line segment, that is, a line segment with one end unchanged and the other end extending indefinitely. Mathematically, the extension of a line segment is also called a ray, which is represented by an arrow. The direction of the extension line of the line segment is the same as that of the line segment. The extension line of a line segment has only one endpoint, and the other end extends infinitely, which is a ray.