Deceleration refers to the changing velocity of the car during a break.  

• In mechanics. acceleration is the rate of change of the velocity with respect to time. For example, when a vehicle is idle and travels in a straight line at increasing speeds, it is acceleration in the direction of travel. If the car will steer to turn an acceleration occurs toward a new direction and changes its motion vector. 
• If the speed of the vehicle decreases, this is an acceleration in the opposite direction and mathematically a negative, and this is referred to as deceleration.