transponder
noun
tran·spon·der tran(t)-ˈspän-dər 
: a radio or radar set that upon receiving a designated signal emits a radio signal of its own and that is used especially for the detection, identification, and location of objects and in satellites for relaying communications signals
Did you know?
This word was coined during World War II by simply joining pieces of the words transmitter and responder. Transponders are basic to modern aviation and communications satellites, and they're finding new uses in fields such as medicine as well. But they're now also part of everyday life. The "E-ZPass" that lets you drive right through turnpike tollbooths is a transponder, and the car you're driving may not even start unless it recognizes the signal from your personal key's transponder. In a big crowded foot race, you may carry a tiny transponder on your shoe that records when you cross both the starting line and the finish line.
Example Sentences
Recent Examples on the Web The transponder antenna will reflect radar signals from Biomass back to the satellite, to help confirm the mission is operating optimally. 
Tim Newcomb, Popular Mechanics, 26 Aug. 2022 The tiny paddles click with satisfying feedback and draw from an internal nine-volt battery to actuate an infrared transponder. Basem Wasef, Car and Driver, 18 Aug. 2022 Carpoolers will need an E-ZPass Flex transponder to use the lanes without paying. Luz Lazo, Washington Post, 18 Aug. 2022 Then the boat turned off the transponder required by international maritime rules, so that its location could no longer be tracked. Evan Osnos, The New Yorker, 18 July 2022 Details on what the pass will be like – a transponder, or a sticker affixed to a windshield – have not been finalized. al, 6 July 2022 The new rates will range from $2.40 per trip for a vehicle with a transponder to $14.38 for a five-axle vehicle paying by mail. Krista Johnson, The Courier-Journal, 22 June 2022 The color scale of aircraft traffic is based on the number of aircraft that sent transponder locations originating from that spot. David Yanofsky, Quartz, 16 June 2022 With a calm voice, the air traffic controller at Treasure Coast International Airport in Florida instructed the passenger how to use his transponder so the aircraft could be located via radar. Nathan Solis, Los Angeles Times, 11 May 2022 See More
Tim Newcomb, Popular Mechanics, 26 Aug. 2022 The tiny paddles click with satisfying feedback and draw from an internal nine-volt battery to actuate an infrared transponder. Basem Wasef, Car and Driver, 18 Aug. 2022 Carpoolers will need an E-ZPass Flex transponder to use the lanes without paying. Luz Lazo, Washington Post, 18 Aug. 2022 Then the boat turned off the transponder required by international maritime rules, so that its location could no longer be tracked. Evan Osnos, The New Yorker, 18 July 2022 Details on what the pass will be like – a transponder, or a sticker affixed to a windshield – have not been finalized. al, 6 July 2022 The new rates will range from $2.40 per trip for a vehicle with a transponder to $14.38 for a five-axle vehicle paying by mail. Krista Johnson, The Courier-Journal, 22 June 2022 The color scale of aircraft traffic is based on the number of aircraft that sent transponder locations originating from that spot. David Yanofsky, Quartz, 16 June 2022 With a calm voice, the air traffic controller at Treasure Coast International Airport in Florida instructed the passenger how to use his transponder so the aircraft could be located via radar. Nathan Solis, Los Angeles Times, 11 May 2022 See More Word History
Etymology
transmitter + responder
First Known Use
circa 1944, in the meaning defined above