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A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal. – Wikipedia1)
Normally it takes power from the trigger source to operate the relay. A latching relay, however, moves from position to position and locks there even when power is removed.