Plain English Meaning

Slowing down. The aircraft is still moving forward, but its speed is decreasing.

Full 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).

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.