Plain English Meaning
It is the practical side of how an aeroplane flies. It looks at the push, pull, lift, and drag working on the aircraft once it is in the air, and how those forces change when the pilot climbs, descends, turns, or changes speed.
Full Definition
The study of how the four forces acting on an aircraft — lift, weight, thrust, and drag — interact during actual flight to produce and control motion through the air. It covers how these forces behave in straight-and-level flight, climbs, descents, turns, and changes in airspeed or configuration, and how the pilot manipulates controls to keep them in the desired balance.