Ultraviolet radiation (or UV) is electromagnetic radiation whose wavelength is shorter than the visible spectrum but longer than X-rays. Most stars emit some amount of ultraviolet radiation and it is generally very harmful to organic life. Consequently, many planets that have evolved organic life often have an ozone layer in the upper atmosphere that absorbs ultraviolet radiation and protects the organisms on the surface. Damage to the ozone layer can result in increased surface levels of ultraviolet radiation, threatening established and developing life.
The shy Tarchannen III species, invisible in and easily frightened by regular light, absorbed ultraviolet light and could so be made visible. Data modified a emergency beacon to operate on UV and so was able to locate Geordi La Forge who was slowly being transformed into one of the beings on the surface of Tarchannen III. (TNG: "Identity Crisis")