Plain English Meaning
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.
Full 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).