Plain English Meaning
An airplane with a strong engine -- more than 200 horsepower. Before you can fly one as pilot in command, an instructor has to train you in it and sign your logbook saying you're qualified.
Full Definition
In FAA regulatory terms, a high performance airplane is one with an engine of more than 200 horsepower. Under 14 CFR 61.31(f), a pilot must receive and log ground and flight training from an authorized instructor in a high performance airplane, and obtain a one-time logbook endorsement, before acting as pilot in command of one.
Context Anchor
Seen in logbook endorsements, aircraft checkout training, and discussions of what a pilot is legally allowed to fly.