Airborne Collision Avoidance System in the context of "Transponder (aeronautics)"

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 air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other.

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