1 An electrical switch that allows a low power to control a higher one. A small current energizes the relay, which closes a gate, allowing a large current to flow through.
2. An electromechanical or solid-state device operated by variations in the input which, in turn, operate or control other devices connected to the output.

They are used in a wide variety of applications throughout industry; such as, in telephone exchanges, digital computers, motor and sequencing controls, and automation systems.

Highly sophisticated relays are utilized to protect electric power systems against trouble and power blackouts; as well as, to regulate and to control the generation and distribution of electrical power.

In private residences, relays are used in refrigerators, automatic washers, dishwashers, and heat and air-conditioning controls.

3. An electromechanical switch operated by a flow of electricity in one circuit and controlling the flow of electricity in another circuit.

A relay composed essentially of an electromagnet with a soft iron bar, called an armature, held close to it.

A movable contact is connected to the armature in such a way that the contact is held in its normal position by a spring and when the electromagnet is energized, it exerts a force on the armature that overcomes the pull of the spring and moves the contact so as to either complete or to break a circuit.