transponder

American  

[tran-spon-der] / trænˈspɒn dər /

Or transpondor

noun

1. a radio, radar, or sonar transceiver that automatically transmits a signal upon reception of a designated incoming signal.

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transponder British  

/ trænˈspɒndə /

noun

1. a type of radio or radar transmitter-receiver that transmits signals automatically when it receives predetermined signals

2. the receiver and transmitter in a communications or broadcast satellite, relaying received signals back to earth

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transponder Scientific  

/ trăn-spŏn′dər /

1. A radio or radar transmitter and receiver that responds to an incoming signal either by broadcasting its own predetermined signal (as in aircraft identification systems) or by relaying the incoming signal at a different frequency (as in satellite communications).

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Etymology

Origin of transponder

First recorded in 1940–45; trans(mitter) + (res)ponder

Explanation

A transponder is a communication device that can either send or receive signals (or both). With a special transponder in your car, you can drive slowly through a tollbooth as your toll is collected electronically. The word transponder is what's known as a portmanteau, or a combination of two words into one — in this case, it comes from transmit and respond. Airplanes carry transponders that can communicate with the radar used by air traffic controllers on the ground, and race cars contain transponders that help time their laps accurately.

Example Sentences

See Examples For:

transponder

Many ships have been "going dark" when crossing the strait, meaning they turn off their transponder to avoid detection.

From BBC ● Aug. 3, 2026

Tracking data shows it switched off its transponder after passing through the Channel two weeks ago, and only reappeared when preparing to dock in Togo.

From BBC ● Aug. 1, 2026

That would be useful particularly if a ship has turned off its location transponder.

From The Wall Street Journal ● Apr. 9, 2026

Homendy added that the runway safety system ASDE-X, designed to track aircraft and ground vehicles, did not generate an alert before the crash because the fire truck was not equipped with a transponder.

From Barron's ● Mar. 24, 2026

The null-gee doughnut that our apartment was spoked into had a supply of escape-jumpers, single-use jet-packs with a simple transponder that screamed for help on all the emergency channels.

From A Place so Foreign by Doctorow, Cory