transponder

[tran-spon-der]
noun
a radio, radar, or sonar transceiver that automatically transmits a signal upon reception of a designated incoming signal.
Also, transpondor.


Origin:
1940–45; trans(mitter) + (res)ponder

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
transponder or transpondor (trænˈspɒndə)
 
n
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
 
[C20: from transmitter + responder]
 
transpondor or transpondor
 
n
 
[C20: from transmitter + responder]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Etymonline
Word Origin & History

transponder
1945, from trans(mit) + (res)pond + -er
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
transponder   (trān-spŏn'dər)  Pronunciation Key 
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).
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Unfortunately the plane's transponder communicates with secondary radar, not
  the primary receiver.
The discrepancy arose because of where the transponder boxes were placed in the
  cars.
He is also developing a tiny acoustic transponder that could be implanted in
  the jelly and used to track it with an unmanned sub.
And transponder signals can be detected by anyone with an appropriate receiver.
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