A relay is an electrically operated switch. It has a set of input terminals for one or more control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.

Relays are used to control a circuit by an independent low-power signal and to control several circuits by one signal. They were first used in long-distance telegraph circuits as signal repeaters that transmit a refreshed copy of the incoming signal onto another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

The traditional electromechanical relay uses an electromagnet to close or open the contacts, but relays using other operating principles have also been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protective relays or safety relays.

Latching relays require only a single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling.

In 1809 an electrolytic relay was designed as an alarm for an electrochemical telegraph by Samuel Thomas von Sömmerring.

Electrical relays got their start mainly in application to telegraphs. American scientist Joseph Henry is often cited to have invented a relay in 1835 in order to improve his version of the electrical telegraph, developed earlier in 1831. However, Henry never published any of these experiments and dating for his relay experiments is based solely on the words of Henry himself and his students, often decades later.

In March 1837 Edward Davy deposited a letter with the British Secretary for the Society of Arts containing his ideas for an electromagnetic relay, which, even if it was not the first, was considered more practical than previous designs, being a ‘make-and-break’ type rather than being based on the use of mercury. He did this two months before Charles Wheatstone and William Cooke filed their first patent for their telegraph system and would file a patent for the same idea a year later.

However, an official patent was not issued until 1840 to Samuel Morse for his telegraph, which is now called a relay. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing signals to be propagated as far as desired.