A device that receives a signal and re transmits it either in the same band or by re-modulating it in a different band.
Repeaters are usually used to boost the range of transmitters. For example, the range of a 5W hand held can be greatly increased. Repeaters designed for duplex operations have different transmit and receive frequency. The repeaters also have to be keyed on by sending certain tone frequency - or a sequence of tones.