1. A device that relays electrical signals not necessarily in the same form or on the same frequency as received or an electrical device designed to receive a specific signal and automatically transmit a specific reply or a radio or radar transceiver that automatically transmits a signal of its own when it receives a predetermined signal from elsewhere, used especially for locating and identifying objects.
2. A blend of transmitter and responder.

• Transmitter: A piece of broadcasting equipment that generates a radio-frequency wave, modulates it so that it carries a meaningful signal, and sends it out from an antenna.
• Responder: In general, an instrument that indicates the reception of an electric or electromagnetic signal.