proper velocity
英[ˈprɔpə viˈlɔsiti]美[ˈprɑpɚ vəˈlɑsɪti]
In relativity, proper-velocity, also known as celerity, is an alternative to velocity for measuring motion. Whereas velocity relative to an observer is distance per unit time where both distance and time are measured by the observer, proper velocity relative to an observer divides observer-measured distance by the time elapsed on the clocks of the traveling object.