Plain English Meaning
How far down the flaps are lowered, measured in degrees from their stowed position. A small deflection means the flaps are barely down; a large deflection means they are lowered a lot.
Full Definition
The angle, measured in degrees, to which the wing flaps are extended downward from their retracted (faired) position. Flap deflection changes the wing's camber and, at larger angles, its frontal area, altering lift and drag characteristics.
Context Anchor
Used when discussing flap settings during takeoff, approach, landing, and go-around planning.
Derivation
Deflection' comes from the Latin 'deflectere,' meaning 'to bend down or aside.' In aviation, it describes how far a has been moved from its — here, how far the has been bent downward.