Average acceleration is the change in velocity divided by an elapsed time. For instance, if the velocity of a marble increases from 0 to 60 cm/s in 3 seconds, its average acceleration would be 20 cm/s$_{2}$. This means that the marble's velocity will increase by 20 cm/s every second.

definition

Define and calculate instantaneous acceleration

When the object is moving with variable acceleration, then the object possesses different acceleration at different instant. The acceleration of the object at different instant of time or at given time of motion, is called instantaneous acceleration.