Rapidity n. : The quality or state of being rapid; swiftness; celerity; velocity; as, the rapidity of a current; rapidity of speech; rapidity of growth or improvement.

Example:
Why does length of day increase with greater Rapidity as one approaches the summer solstice?

Taxonomy
rapidity, speed, free encyclopedia, History, high speeds, 1914, cosh, accelerated, motion, frame of reference, speed of light, Lorentz transformation, theories, relativity physics, search, proportional, hyperbolic angle, artanh.

Defination from the web:
rapidity φ arises in the linear representation of a Lorentz boost as a vector-matrix product ... Therefore the rapidity of an object in a given frame can be viewed simply as the velocity of that object as would be calculated non ...