Acceleration is the rate of change of velocity.

Here's a breakdown:

* Velocity: How fast an object is moving *and* in what direction.

* Acceleration: How quickly the velocity is changing (either in speed or direction, or both).

Examples:

* A car speeding up: Its velocity is increasing, so it's accelerating.

* A car slowing down: Its velocity is decreasing, so it's also accelerating (in this case, it's decelerating).

* A car turning a corner: Even if its speed stays the same, its direction is changing, meaning its velocity is changing, and it's therefore accelerating.