Source : Webster's Revised Unabridged Dictionary (1913)

Rapidity \Ra*pid"i*ty\, n. [L. rapiditas: cf. F. rapidit['e].]
   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 : WordNet®

rapidity
     n : a rate that is rapid [syn: {celerity}, {quickness}]