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.
In the radio world, a relay is a device that receives a signal from a low-power or distant transmitter and retransmits it on the same or different frequency in order to increase the coverage area. For example, the signal from a broadcast facility in a valley would only propagate within that valley. A relay site at the top of a nearby mountain would rebroadcast the original signal to a wider audience. In commercial TV and radio, these relay sites are known as "translators."