A change in speed or direction, or both, is called acceleration.

Here's why:

* Speed: Acceleration happens when an object speeds up (positive acceleration) or slows down (negative acceleration, also called deceleration).

* Direction: Acceleration also occurs when an object changes direction, even if its speed remains constant. Think of a car turning a corner at a steady speed.

In summary: Acceleration is a measure of how much an object's velocity changes over time. Velocity includes both speed and direction.