A ray is a beam of light or radiation. Even on cloudy days, you sometimes see a ray of sunlight shine through the clouds.
In physics, a ray is a line or column of light, heat, or electromagnetic radiation (like an x-ray), while in math a ray is a line that passes through a specific point. A marine biologist would define a ray as a big fish that's related to sharks and has a wide, flat body and a long tail. The root of the first two kinds of rays is the Latin word radius, "spoke" or "staff."