Definition
The four-digit numerical codes, each digit ranging from 0 through 7, that a pilot sets into the aircraft's transponder so air traffic control radar can identify and track the aircraft. Specific codes are assigned for particular purposes: 1200 for VFR flight in the United States, 7500 for unlawful interference (hijack), 7600 for loss of radio communications, and 7700 for general emergency. Other codes are assigned by ATC for individual flights.
Plain English
Four-number settings a pilot dials into the aircraft's radar identification box so controllers on the ground know which blip on their screen is your aircraft. Some numbers have fixed meanings (like the emergency code), and others are given to you by air traffic control.
Context Anchor
You encounter transponder codes when air traffic control tells you to “squawk” a number, when flying VFR on the standard code 1200, or when setting special codes for emergency situations.
Derivation
"Transponder" combines "transmitter" and "responder" — a device that responds to a radar interrogation by transmitting a coded reply. The "code" is the four-digit number it transmits back.
Why Pilots Care
Correct codes let ATC track and separate traffic safely. Using the wrong code or failing to change to an assigned one can cause loss of radar identification or delayed emergency response.
Analogy
A transponder code works a little like a temporary name tag on a radar screen. The airplane is still the same airplane, but the code helps the controller know which target belongs to you.
Intuition Check
Do not think of a transponder code as a password or radio frequency. In this context, it is a four-digit identification or alert number sent by the aircraft transponder.
Example Sentence 1
Before takeoff on a VFR flight, the pilot set the transponder code to 1200 as required in U.S. airspace.
Example Sentence 2
ATC assigned a discrete code of 4523, which the pilot entered to receive radar flight following.