What Is a Change in Position With Respect to a Reference Point?

Motion is defined as a change in position with respect to a reference point. A reference point is a requirement for any description of motion as there is no real fixed point within the universe. This is the basis for Albert Einstein's Theory of Relativity.

Since a reference point is required for motion, all motion is relative. For example, if a person is standing still and sees a bus drive by, it is moving with a speed that is in respect to that person. However, if that person is in a moving car and observes the same bus, its speed is different since its reference point has changed.