LED
Definition
Light-Emitting Diode — a semiconductor that converts electricity directly into light, far more efficiently than bulbs.
Technical definition
A p-n junction diode that emits photons when forward-biased electrons recombine with holes across the band gap. Photon color set by band gap energy. Roughly 10× more efficient than incandescents; enables modern lighting and displays.
Example
A 10 W LED bulb matches a 60 W incandescent — the incandescent wastes ~90% of its energy as heat, the LED converts most input to light.
Common confusion
LEDs are not efficient because they are ‘cold’ — they are efficient because electroluminescence skips the wasteful heat-a-filament step. They still need heat sinks at high power; the physics just wastes far less.