RAPIDITY
celerity, quickness, rapidity, rapidness, speediness
(noun) a rate that is rapid
Source: WordNet® 3.1
Etymology
Noun
rapidity (countable and uncountable, plural rapidities)
speed, swiftness; the condition of being rapid
(physics) A measure of velocity relative to the speed of light
(physics) A measure of the velocity of a particle in a beam relative to the beam's axis
Source: Wiktionary
Ra*pid"i*ty, n. Etym: [L. rapiditas: cf. F. rapidité.]
Definition: The quality or state of being rapid; swiftness; celerity;
velocity; as, the rapidity of growth or improvement.
Syn.
– -- Rapidness; haste; speed; celerity; velocity; swiftness;
fleetness; quickness; agility.
Source: Webster’s Unabridged Dictionary 1913 Edition