UV light, short for ultraviolet light, is light with a wavelength shorter than that of visible light, but longer than x-rays. This means electromagnetic waves with a wavelength between 400 nm and 10 nm, with energies from 3 eV to 124 eV. UV light is more energetic than visible light and has a shorter wavelength, letting it penetrate more readily through obstacles. The "ultraviolet" in ultraviolet light references that UV light is beyond violet on the electromagnetic spectrum. Like other forms of electromagnetic radiation outside the visible spectrum, UV light is invisible, but it can be observed indirectly by the way it makes many other substances fluoresce in the visible spectrum.

In popular culture, UV light is primarily thought of as a party light because of the way it makes textiles and clothing, particularly white shirts, fluoresce brightly. "Black lights" are synonymous with UV light. These lights primarily produce light in the UV portion of the spectrum, but they also produce a slight violet glow. Special posters or other works of art are often created with the express purpose of fluorescing a certain way under a black light.

UV light has many other applications outside of the party scene. It is frequently used in security. For instance, sensitive documents, such as currency, driver's licenses, credit cards or passports, have invisible symbols on them that light up only in the presence of UV light. These are difficult for counterfeiters to copy.

Common fluorescent lamps are powered by UV light. UV light is produced by ionizing low-pressure mercury vapor, which is then absorbed by a special fluorescent coating, which in turn produces visible light. Fluorescent lights are more energy-efficient than conventional light bulbs.

Biologists and zoologists are quite fond of UV light as it helps them take nighttime organism surveys in the field. Certain birds, reptiles, and insects (such as bees) are clearly visible under UV light, and quickly flashing a UV light over a small area can allow observers to count the approximate number of organisms of a given type in that area. This is very helpful because many animals are highly nocturnal and rarely if ever seen during the day.

Besides the above mentioned applications, UV light can also be used for spectrophotometry (to analyze chemical structure), analyzing minerals, chemical markers, photochemotherapy (for psoraisis), very fine resolution photolithography, checking electrical insulation, sterilization, disinfecting drinking water, food processing, lasers, and many other areas.