rapidity

Etymology

rapid + -ity, from French rapidité, from Latin rapiditas.

noun

1. speed, swiftness; the condition of being rapid
   Although appreciating the rapidity and frequency of the Southern electric services I was now to use on short journeys, I became more than ever convinced that electric traction offers very little of interest to the dyed-in-the-wool railwayist. 1944 November and December, Talisman, “A Broadening Horizon”, in Railway Magazine, page 340
2. (physics) A measure of velocity relative to the speed of light
3. (physics) A measure of the velocity of a particle in a beam relative to the beam's axis