A switch or multiple switches that is operated by an electromagnet. A relay lets you use one circuit to control another, often so a low voltage or low current signal can switch a higher voltage or power. A high-power relay used for switching line power is often called a contactor. Some relay modules [often just called relays] include decoders to allow control via DMX or some other serial protocol. A solid-state relay replaces the mechanical switch and electromagnet with an optocoupler and a triac.