Definition of "rapidity" in English

noun

1. speed, swiftness; the condition of being rapid

2. (relativity) 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. (physics) A measure of the velocity of a particle in a beam relative to the beam's axis