Angle of attack

The acute angle between the chordline of a propeller blade and the relative wind. The angle of attack is affected by both the engine rpm and the forward speed of the aircraft.

Aviation Maintenance Technician Handbook — Powerplant, p. G-2

The angle between the airfoil’s chord line and the relative wind.

Helicopter Flying Handbook, p. G-1

The angle of attack is the angle at which relative wind meets an airfoil. It is the angle that is formed by the chord of the airfoil and the direction of the relative wind or between the chord line and the flight path. The angle of attack changes during a flight as the pilot changes the direction of the aircraft and is related to the amount of lift being produced.

Pilot's Handbook of Aeronautical Knowledge, p. G-3