A change in velocity which happens when something speeds up, slows down, or moves in a different direction: Acceleration is the rate of change in speed with respect to time.

The rate of change of a moving body's velocity which has two components: speed and direction: An object's acceleration is altered by either a change in speed, a change in direction, or both.

The acceleration is due to gravity, for instance, 32 feet (9.8 meters) per second; means that for every second an object falls, its velocity is increasing, too.