Definition
To reduce speed quickly and deliberately, typically through aggressive aerodynamic braking, prompt power reduction, wheel braking, or a combination of these, in order to slow the aircraft within a short distance or short period of time.
Plain English
To slow the airplane down fast, on purpose, using whatever combination of reduced power, drag, and brakes the situation calls for.
Context Anchor
Seen in discussions of landing rollout, stopping on the runway, and situations where the airplane must lose speed quickly while staying under control.
Derivation
From Latin 'de-' (down, away from) and 'celer' (swift) -- literally 'to take swiftness away.' 'Rapidly' simply emphasizes that the slowing happens quickly rather than gradually.
Why Pilots Care
Determines whether the aircraft can stop within the remaining runway during a high-speed abort.
Grounding Statement
Picture the airplane after the wheels are on the runway: its speed is coming down quickly as braking and air resistance slow it.
Intuition Check
Rapidly does not mean carelessly, harshly, or as hard as possible. In flying, it means the speed is decreasing quickly while the pilot still keeps the airplane under control.
Example Sentence 1
After the tower called for an immediate stop, the pilot decelerated rapidly by closing the throttle, raising the flaps to put weight on the wheels, and applying firm, even braking.
Example Sentence 2
The aircraft must be capable of decelerating rapidly if an abort is initiated before reaching V1.