Plain English Meaning
A radio device on the aircraft that answers radar pings from air traffic control and tells them both who you are and how high you are flying.
Full Definition
An airborne radio transponder equipped with the Mode C function, which automatically reports the aircraft's pressure altitude to air traffic control radar in addition to its assigned four-digit identification code. When interrogated by a secondary surveillance radar, it replies with both the squawk code and altitude data, allowing controllers to see the aircraft's position and altitude on their radar display.