Light in the near ultraviolet range, just short of visible light. Fluorescent penetrants absorb this ultraviolet radiation and emit light in the visible region. The darker the surroundings the brighter this emitted light appears to be. Extended exposure to black light can be harmful unless protective eyewear is used. See Black Lights
An item that is "black light sensitive" will fluoresce, or glow when exposed to ultraviolet light (black light). These lights are often seen in nightclubs or on stages, and have a purplish color to the eye.
Black light (also Wood's light) is the common name for a lamp emitting electromagnetic radiation that is almost exclusively in the soft near ultraviolet range, and very little visible light. In the United Kingdom this type of lighting is more commonly referred to as simply "UV light".