The rate of motion at which a body changes position is called velocity.

Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* Speed simply tells you how fast something is moving, but velocity tells you how fast something is moving *and* in what direction.

* Therefore, velocity accurately describes the change in position over time, making it the appropriate term for the rate of motion.

Let me know if you'd like to learn more about velocity, speed, or other related concepts!