Definition
A four-digit number, with each digit ranging from 0 to 7, set on the aircraft's transponder so that ATC's radar can identify and track the specific aircraft. The code is assigned by ATC or, in some cases, is a standard code reserved for a particular situation (such as 1200 for VFR flight in the United States, 7500 for hijacking, 7600 for lost communications, and 7700 for emergency).
Plain English
A four-number setting the pilot dials into a small radio device on the aircraft so that air traffic controllers can pick out that specific aircraft on their radar screen.
Context Anchor
You encounter transponder codes when setting the aircraft transponder before departure, after receiving radar service, or when a controller assigns a new code in flight.
Derivation
Transponder' is a blend of 'transmitter' and 'responder' -- a device that responds with a transmission when interrogated by ground radar. The 'code' is simply the number it transmits back, which tells the controller which blip on the screen is which aircraft.
Why Pilots Care
Correct use prevents misidentification, supports traffic separation, and allows efficient routing and altitude assignments.
Intuition Check
A transponder code is not a secret password or a radio frequency. It is a four-digit identification number entered into the transponder so radar equipment can associate a radar target with the correct aircraft.
Example Sentence 1
After contacting departure control, the pilot was told to 'squawk 4271,' so she dialed that transponder code into the panel.
Example Sentence 2
ATC assigned the transponder code 4523 after radar contact was established.