Deceleration means slowing down. When a car approaches a red light and the driver presses the brake pedal, the car decelerates. When a sprinter crosses the finish line and gradually stops running, that's deceleration too.

If you're riding your bike down a hill and gaining speed, you're accelerating. When you squeeze the brakes and slow down, you're decelerating.

Scientists and engineers use this word precisely when measuring how quickly something slows down. A roller coaster decelerates as it climbs the next hill. A spacecraft decelerates when it fires its reverse thrusters. Your heart rate decelerates after you finish exercising and start to rest.

In everyday conversation, people usually just say “slowing down” or “braking,” but deceleration is the technical term. It's especially useful when you need to be exact about the rate of slowing, like when engineers design car brakes or pilots calculate landing distances.