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Acceleration is a measure of the change in velocity of a moving object. It shows how quickly velocity changes. Acceleration may reflect a change in speed, a change in direction, or both. Because acceleration includes both size (speed) and direction, it is a vector. People commonly think of acceleration as in increase in speed, but a decrease in speed is also acceleration. In this case, acceleration is negative. Negative acceleration is called deceleration.
A change in direction without a change in speed is acceleration as well.
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