Acceleration:

Acceleration is an expression of the change in the velocity of an object. This can take place as a change in direction, a change in speed, or both. Acceleration can be defined in one dimension (all along a straight line), in 2-dimensions (within a flat plane), or in 3- dimensions (in space), merely as can velocity. Acceleration at times occurs in similar direction as an object's velocity vector, though this is not essentially the case.