Definition
A reduction in the rate of motion of an aircraft, expressed as a negative change in velocity over time. In flight, deceleration occurs when the forces acting against motion (drag, reverse thrust, braking) exceed the forces producing motion (thrust, gravity along the flight path).
Plain English
Slowing down. The aircraft is still moving forward, but its speed is decreasing.
Context Anchor
You will see this term when reading about changes in airspeed during basic flight maneuvers, approaches, landings, and power changes.
Derivation
From the Latin 'de-' meaning 'down from' or 'away,' combined with 'celer' meaning 'swift.' Literally 'taking away from swiftness' — reducing speed.
Why Pilots Care
Directly affects landing distance, brake wear, and the ability to stop safely within the available runway.
Intuition Check
Deceleration does not only mean braking on the ground. In aviation, it means any slowing of the aircraft, including slowing in flight when power is reduced or drag increases.
Example Sentence 1
After touchdown, the pilot applied the brakes and used aerodynamic drag to achieve smooth deceleration along the runway.
Example Sentence 2
During the approach, reducing power produced a noticeable deceleration before the flare.