While "X-ray light" or "radio light" have never been used in any widespread terminology, the existence of "invisible light" has become known at the beginning of the 19th century, when the terms "infrared light" and "ultraviolet light" have been coined for it.

"Invisible light" is light that is invisible for humans, but which is visible for many animals and for many of the devices that are designed to detect visible light. Traditionally, "invisible light" was distinguished from X-rays (with higher frequency) and radiant heat (with lower frequency), because those required different kinds of emitting and detecting devices than those designed for light.