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

It said another factor was the lack of a transponder on the fire engine.

From Barron's • Apr. 23, 2026

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

From The Wall Street Journal • Apr. 9, 2026

The ship was off Muscat, Oman, early Friday, still broadcasting the message "owner France" on its transponder system in the field usually used to give the destination.

From Barron's • Apr. 3, 2026

On Monday, Panama-flagged cargo ship Guan Yuan Fu Xing was the latest to make it safely through the Strait of Hormuz, two days after changing its destination to "CHINA OWNER" via its AIS transponder.

From Barron's • Mar. 9, 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