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countable and uncountable, plural rapidities
speed, swiftness; the condition of being rapid quotations examples
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
(physics) A measure of velocity relative to the speed of light examples
(physics) A measure of the velocity of a particle in a beam relative to the beam's axis examples