To enter a four-digit octal code into an aircraft's transponder so air traffic control can identify the flight on their radar screens. Controllers assign discrete codes to individual flights, while standard codes indicate visual flight rules or emergencies like radio failure and hijacking. As a noun, mechanics also use it to mean a written maintenance defect.
Cessna Three Alpha Bravo, squawk zero three four five and ident.
Never squawk 7500, 7600, or 7700 unless you are actually experiencing a hijacking, comms failure, or general emergency.
Related terms
Radar cross section
A measure of how detectable an aircraft or object is to radar systems.
Hand-off
The transfer of radar identification and radio communication for an aircraft from one air traffic controller to another.
Approach
The final phase of a flight where an aircraft aligns with the runway for landing.