A ray is part of a line that starts at one point and extends infinitely in a particular direction. This is a key concept that students must grasp in order to understand and measure angles, as every angle is made up of two rays. Rays are also important to know for other applications, such as lasers, which emit light in straight, one-way lines from their source.

There are many different ways to define a ray, but all of them include the idea that it has a unidirectional length. This is different from other geometric figures, such as a circle, which has both a length and a radius. The arrowhead at the end of the ray indicates the direction it is moving, as well.

The crucial elements to identify a ray are its endpoint and its direction of extension. The endpoint is the fixed location from which the ray originates and serves as the starting point or anchor for the ray. The direction of the ray is indicated by an arrowhead, emphasizing the path that the ray extends infinitely along.

Rays are often shown as lines with an arrow head at the end, but they are actually one-dimensional figures. This means they have no width, unlike a line or a line segment, which can be measured with a ruler. They may pass through other points on their endless journey, as shown in the figure below, where the ray from A passes through B. If both rays start at the same point, they are called identical rays.