* Moving at a constant speed in a circle: This is called centripetal acceleration. Even though the speed is constant, the direction of motion is changing, which means the velocity is changing, and therefore there is acceleration.

* Increasing its speed: This is the most common way we think about acceleration.

* Decreasing its speed: This is often called deceleration, but it's still a type of acceleration. Acceleration is simply a change in velocity, and that can be an increase, decrease, or change in direction.

* Changing direction: Even if an object's speed is constant, if its direction is changing, it is accelerating.

Key takeaway: Acceleration means there is a change in velocity, which can be a change in speed, direction, or both.