DECELERATION
deceleration
(noun) the act of decelerating; decreasing the speed; “he initiated deceleration by braking”
deceleration, slowing, retardation
(noun) a decrease in rate of change; “the deceleration of the arms race”
deceleration
(noun) (physics) a rate of decrease in velocity
Source: WordNet® 3.1
Noun
deceleration (countable and uncountable, plural decelerations)
(uncountable) The act or process of decelerating.
(countable) The amount by which a speed or velocity decreases (and so a scalar quantity or a vector quantity).
Antonyms
• (act or process of decelerating): acceleration
Source: Wiktionary