* Change in speed: Acceleration means the object is either speeding up or slowing down.

* Change in direction: Even if the speed remains constant, a change in direction indicates acceleration (think of a car going around a curve).

* Change in both speed and direction: This is the most common case, like a car accelerating forward and then turning.

* Force acting on the object: Acceleration is caused by a force. Newton's second law of motion states that the acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass (F = ma).

Important Note: An object can be moving at a constant speed and still be accelerating if its direction is changing. A classic example is an object moving in a circle at a constant speed.