When a vehicle or machine decelerates or when someone in a vehicle decelerates, the speed of the vehicle or machine is reduced. the sensation of the train decelerating. accelerate + deceleration de·cel·era·tion The harder the brake pedal is pressed, the greater the car's deceleration. acceleration
When the rate of something such as inflation or economic growth decelerates, it slows down. Inflation has decelerated remarkably over the past two years. accelerate + deceleration de·cel·era·tion a significant deceleration in the annual rate of growth. acceleration. to go slower, especially in a vehicle = slow down accelerate