Plain English Meaning
Learning how to spot when an airplane is about to get away from you, how to stop that from happening, and how to get it back under control if it does.
Full Definition
Training and techniques aimed at recognizing the early signs of an aircraft departing from intended flight attitudes, preventing that departure when possible, and returning the aircraft to controlled flight if it occurs. An upset is generally defined as an unintentional pitch attitude greater than 25° nose-up, greater than 10° nose-down, a bank angle greater than 45°, or flight at airspeeds inappropriate for the conditions.