part of a line that starts at one endpoint and extends forever in one direction is a(n):

A part of a line that starts at one endpoint and extends forever in one direction is called a ray. In geometry, a line is a straight one-dimensional figure that has no thickness and extends infinitely in both directions. A ray, however, has a defined starting point, known as the endpoint, and extends infinitely in one direction from this point. This distinction is essential: while a line can be thought of as a collection of points stretching endlessly in both directions, a ray includes only the points on one side of its starting point, continuing infinitely beyond it.

This concept is fundamental in geometry, as it sets the foundation for understanding angles, intersections, and various geometric constructions. When you represent a ray, you often denote it using two points: the endpoint and another point on the ray, such as $\overrightarrow{AB}$, where point A is the starting point and point B is any other point on the ray extending from A.