Acceleration is a change in velocity. Velocity includes both speed and direction. So, acceleration can:

* Speed up an object: This is the most common example of acceleration.

* Slow down an object: This is also acceleration, but it's often called deceleration.

* Change the direction of an object: Even if an object maintains the same speed, if its direction changes, it's accelerating.

Think about a car turning a corner at a constant speed. It's still accelerating because its direction is changing.