deceleration

Deceleration is a slowing down: the unexpected deceleration of your car might mean that you've run out of gas.

Use the noun deceleration when you talk about a gradual decrease in speed. When you're on a Ferris wheel, its deceleration tells you that the ride is almost over, and the deceleration of a train at a railroad crossing happens for safety reasons. The opposite of deceleration is acceleration, or speeding up. The word deceleration was originally an 1890's railroad term, constructed by adding de, or "the opposite of" to acceleration.

Definitions of deceleration
1

n a decrease in rate of change

“the deceleration of the arms race”
Synonyms:
retardation, slowing
Antonyms:
acceleration
an increase in rate of change
Type of:
alteration, change, modification
an event that occurs when something passes from one state or phase to another

n the act of decelerating; decreasing the speed

“he initiated deceleration by braking”
Antonyms:
acceleration, quickening, speedup
the act of accelerating; increasing the speed
Type of:
hurrying, speed, speeding
changing location rapidly

n (physics) a rate of decrease in velocity

Antonyms:
acceleration
(physics) a rate of increase of velocity
Type of:
rate
a magnitude or frequency relative to a time unit