Acceleration

Acceleration is the time rate of change of velocity, just as velocity is the time rate of change of position.

Instantaneous acceleration can be calculated as the time rate of change (derivative) of velocity:  $$\vec{a} = \frac{\delta \vec{v}}{\delta t}$$
Average acceleration can be calculated as a change in velocity:  $$\vec{a}_{avg} = \frac{\Delta \vec{v}}{\Delta t}$$