The quantity that represents the rate of change of an object's position is called velocity. Velocity is defined as the change in position (displacement) over the change in time. It can be expressed mathematically as:
\[ v = \frac{\Delta x}{\Delta t} \]
where \( v \) is velocity, \( \Delta x \) is the change in position, and \( \Delta t \) is the change in time. Velocity is a vector quantity, meaning it has both magnitude and direction. If only the magnitude is considered, it is referred to as speed.