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A transponder (short-for transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on radar and on other aircraft's collision avoidance systems.

Air traffic control units use the term "squawk" when they are assigning an aircraft a transponder code, e.g., "Squawk 7421". Squawk or squawking thus can be said to mean "select transponder code" or "I have selected transponder code xxxx".