When an object's velocity is changing, it is said to be accelerating. In this case, the average velocity over a time interval (in general) is not equal to the instantaneous velocity at each instant in that time interval. So how do we determine the instantaneous velocity?
By investing Illustration 2.3, we will find that the instantaneous velocity is equal to the slope of the position versus time graph at a specific time. By making the time interval smaller and smaller, the average velocity approaches the instantaneous (as we approach an "instant of time").