A relay refers to a device used in electrical or electronic circuits for the protection of devices connected to these circuits. It is a critical device and is referred to as a silent sentinel. One of its popular functions is to connect or disconnect the device from the mains circuit. In power circuits, the primary device protects the circuit and the equipment connected to it from overloads, short circuits, earthing and surges due to either switching or lightening.

Earlier relays were electromechanical in construction. They have now been replaced by static relays. Miniature versions are commonly used in communication and computer circuits.