In a graph describing the velocity of an object, the slope of secant line on the graph represents average velocity, while the slope of tangent line on the graph represents instantaneous velocity. Actually instantaneous velocity is a special case of average velocity. Based on the concepts in calculus, instantaneous velocity can actually be interpreted as a limit of average velocity.

When time difference (∆T) gets smaller and smaller and tends to be zero, the average velocity between these two extremely small time difference can be regarded as the instantaneous velocity at that point. The smaller the distance, the smaller the difference between average velocity and instantaneous velocity will be.