Depending on which level you are studying acceleration, you may or may not be expected to calculate instantaneous acceleration, as it requires basic knowledge of calculus. Nonetheless, I will briefly define instantaneous acceleration.
The term “instantaneous" is defined as occurring in an instant. Therefore, instantaneous acceleration is simply the acceleration of an object at an instant. Although this may seem to be contradicting the concept of average acceleration, which always considers two points in time, it actually isn’t. And this is where the calculus comes in.
One of the first concepts you learn in calculus is limits. In relation to acceleration, if we were to consider point A as introduced earlier, and then take point B to be closer and closer to A so that the average acceleration is calculated over shorter and shorter time intervals, we will get the limit of the average acceleration as the time interval approaches zero. And this is precisely instantaneous acceleration.
However, a more practical approach to calculating instantaneous acceleration is using derivatives. This also requires some basic knowledge of calculus. With this it is obvious that instantaneous acceleration is equal to the rate of change of instantaneous velocity with respect to time.