From AMS Glossary
An imaginary bundle of propagating electromagnetic or acoustic energy, the lateral dimensions of which are negligible.
It is impossible to isolate a ray. Nevertheless, rays are useful conceptual devices if used with a knowledge of their limitations. For example, the rainbow can be described by imagining sunlight incident on a raindrop to be subdivided into many rays, each of which obeys the laws of (specular) reflection and refraction. Because rays do not exist, ray optics (or geometrical optics) is an approximation.