rapidity

Short definition from Wiktionary

Noun

1. 1
   speed, swiftness; the condition of being rapid
2. 2
   A measure of velocity relative to the speed of light; defined as artanh(v), where artanh is the hyperbolic arctangent, and v = speed (with c = speed of light = 1).
3. 3
   A measure of the velocity of a particle in a beam relative to the beam's axis