What is Definition of a high performance aircraft?

A high-performance airplane is an airplane with an engine of more than 200 horsepower.

For example, a standard Cessna 172 is not a high performance airplane. A Cessna 182 generally is.

An extra endorsement by a flight instructor is required before a pilot is legally authorized to fly such an airplane.

Here it is in FAA Legalese:

(1) Except as provided in paragraph (f)(2) of this section, no person may act as pilot in command of a high-performance airplane (an airplane with an engine of more than 200 horsepower), unless the person has--

(i) Received and logged ground and flight training from an authorized instructor in a high-performance airplane, or in a flight simulator or flight training device that is representative of a high-performance airplane, and has been found proficient in the operation and systems of the airplane; and

(ii) Received a one-time endorsement in the pilot's logbook from an authorized instructor who certifies the person is proficient to operate a high-performance airplane.