What is the definition of velocity?

The definition of velocity encompasses several aspects related to motion and is more comprehensive than just how fast something is moving. While "how fast something is moving" refers primarily to speed, velocity specifically incorporates both the speed of an object and its direction of motion. In physics, velocity is defined as the rate of change of an object's position with respect to time, which includes a directional component. This is why "the rate of change of position" also correctly describes velocity, but it is the inclusion of direction that distinctly categorizes it as velocity rather than speed alone.

Reflecting on the other options, "how quickly something changes direction" describes the concept of acceleration but does not represent velocity itself. "The speed of light in a vacuum" is a constant value denoting the maximum speed that light can travel, which is unrelated to the general definition of velocity as it pertains to objects in motion. Therefore, while "how fast something is moving" captures a portion of the definition, understanding velocity requires acknowledgment of both speed and the direction of movement.