Note the Physics:

• In principle, an object's instantaneous velocity is its average velocity over a vanishingly short span of time.
• In practice, we can find the instantaneous velocity by finding the average velocity over a very short time span, since its velocity is approximately constant during a short time interval.
• Over a very short time interval the acceleration of an object is also approximately constant. Thus, instantaneous acceleration is essentially the same as the average acceleration over a very short time interval.