rapidity

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

• IPA^(key): /ɹəˈpɪdɪti/

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rapidity (countable and uncountable, plural rapidities)

1. speed, swiftness; the condition of being rapid
   • 1944 November and December, Talisman, “A Broadening Horizon”, in Railway Magazine, page 340:

     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.

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

• midrapidity
• pseudorapidity