1. A change in velocity: Acceleration is the rate of change of velocity. This means the object's speed or direction must be changing.

* Speeding up: If an object is moving faster, it's accelerating.

* Slowing down: If an object is moving slower, it's also accelerating (this is sometimes called deceleration).

* Changing direction: Even if an object maintains a constant speed, changing direction means its velocity is changing, and therefore it's accelerating.

2. A time interval: Acceleration is measured over a specific period of time. You need to know how long it took for the velocity change to occur.

In summary: Acceleration is the change in velocity over a given time interval. You need to have both a change in velocity and a time frame to express acceleration.