Plain English Meaning
An airplane whose engine produces more than 200 horsepower. To fly one as pilot in command, a pilot must first get a special sign-off in their logbook from an instructor, showing they have been trained on the differences these airplanes bring.
Full Definition
An airplane with an engine of more than 200 horsepower, as defined by the FAA in 14 CFR 61.31(f). Acting as pilot in command of such an aircraft requires a one-time logbook endorsement from an authorized instructor certifying the pilot has received and demonstrated proficiency in ground and flight training specific to high-performance airplanes.