Plain English Meaning
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.
Full 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.