http://en.wikipedia.org/wiki/Relative_velocity
wikipedia 用了個專業術語 relative velocity, 而不是relative speed。'
而relative velocity 是專業術語,但不是狹義相對論的專用術語。。經典力學也能用。
在special relativity theory 那段中 有段有趣的描述:
Whereas the maximum difference speed between two objects is 2c, the maximum absolute value of a relative velocity is equal to c.