Plain English Meaning
A check, done at least every two years, that proves the aircraft's transponder is still working accurately enough to be used in controlled airspace.
Full Definition
A required inspection and test of the aircraft's transponder system, performed every 24 calendar months by an authorized technician under 14 CFR 91.413, to confirm the unit transmits correct identification and altitude data to air traffic control radar.
Context Anchor
You encounter this during preflight planning when checking required aircraft inspections and maintenance records before .