A light-emitting diode, usually called an LED (/ɛl.iː.ˈdiː/) , is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction, as in the common LED circuit. This effect is a form of electroluminescence.

An LED is usually a small area light source, often with extra optics added to the chip that shapes its radiation pattern. LEDs are often used as small indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. LEDs can also be used as a regular household light source. One of the most important applications for society is the sterilization of water and disinfection of devices. Development of a new water sterilization device with a 365 nm UV-LED, Medical and Biological Engineering and Computing, Volume 45, Number 12 / December, 2007