When the acceleration is negative ( but the velocity does not change direction ) , it is sometimes called deceleration . In common speech , the term acceleration is used for an increase in speed ( the magnitude of velocity ) ; a decrease in speed is called deceleration . In physics , a change in the direction of velocity also is an acceleration : for rotary motion , the change in direction of velocity results in centripetal ( toward the center ) acceleration ; whereas the rate of change of speed is a tangential acceleration .