rapidity

noun

Etymology: From rapid + -ity, from French rapidité, from Latin rapiditas.

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.”

   “In clinical practice, the generation of diagnostic hypotheses occurs especially readily and rapidly in subdisciplines such as dermatology, where gross appearance of lesions is of particular importance diagnostically[…]. The primacy of macroscopy in paleopathology means that the activation of skeletal disease scripts occurs with similar facility and rapidity when skeletal remains are examined.”

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. A measure of the velocity of a particle in a beam relative to the beam's axis