Light Emitting Diode. A solid-state, semiconductor device that converts electrical energy directly into light. LEDs show up in Christmas lighting in two contexts: the first is as a power or signal indicator in controllers or SSRs, while the second is their use as a substitute for incandescent lamps. LED Christmas light strings use about one-tenth the energy of an incandescent lamp and have a crystal-clear color brightness that incandescents cannot achieve.