A ray of light given off by a light source is simply called a ray of light or light ray.

Here's why there isn't a specific name for it:

* Light travels in straight lines: Rays of light are used to represent the straight-line path that light travels.

* Multiple rays: A light source emits many rays of light in all directions, not just one.

If you want to be more specific, you could talk about:

* Beam of light: A collection of parallel light rays.

* Light cone: The shape formed by the light rays emanating from a point source.