To decelerate means to slow down or reduce speed. When a car decelerates, the driver eases off the gas pedal or presses the brakes, making the vehicle move more slowly. When a roller coaster decelerates as it approaches the station, you feel yourself pressing forward slightly in your seat as the ride loses speed.

A runner decelerates as they cross the finish line. A spinning top decelerates gradually until it wobbles and falls over. Scientists use the word when describing objects losing velocity: a spacecraft decelerates as it enters a planet's atmosphere, or a baseball decelerates as air resistance slows it.

You might also hear deceleration used more broadly to describe anything slowing its pace. A company's growth might decelerate if sales increase more slowly than before. Your reading speed might decelerate when you hit a particularly difficult passage in a book. The key idea is always the same: something that was moving or progressing at a certain rate begins to slow down.