Plain English Meaning
How much the airplane's airspeed drifts above or below the speed the pilot is trying to hold.
Full Definition
Departures of the airplane's indicated airspeed from a target or assigned airspeed during flight, particularly relevant in large airplanes where slow-speed handling, momentum, and engine spool-up time make precise speed control more demanding.
Context Anchor
Seen during slow-speed flight, approaches, and training discussions where a pilot must hold a precise airspeed in a large airplane.
Derivation
Deviation comes from the Latin 'deviare,' meaning to turn the road. An airspeed deviation is simply the airspeed wandering off its path.