Instantaneous speed

For a straight graph, the gradient is unchanged, so average speed and instantaneous speed are equal. For a curved graph, they are generally different: the chord gives an average across an interval, whereas a tangent gives the speed at one moment.

The speed at one particular moment is its instantaneous speed. During non-uniform motion, instantaneous speed varies. An average speed instead gives one representative rate for the journey as a whole: it answers the question, “If the same total distance had been covered steadily in the same total time, what would that steady speed have been?”