deceleration

Noun

• the act of decelerating; decreasing the speed
  • he initiated deceleration by braking
    

• (physics) a rate of decrease in velocity

• a decrease in rate of change
  • the deceleration of the arms race
    

Related words

• Synonyms: 
  slowing , retardation
• Antonyms: 
  acceleration , quickening , speedup