Acceleration is the rate of change of velocity with time.
In everyday English, the word acceleration is often used to describe a state of increasing speed. For many Americans, their only experience with acceleration comes from car ads. This example illustrates acceleration as defined above, but acceleration in physics is much more than just increasing speed. Any change in either the speed or direction of an object is considered acceleration. Thus, a falling apple accelerates, a car stopping at a traffic light accelerates, and the moon accelerates as it orbits the earth.