The Collaborative International Dictionary of English v.0.48:

Rapidity \Ra*pid"i*ty\ (r[.a]*p[i^]d"[i^]*t[y^]), n. [L.
   rapiditas: cf. F. rapidit['e].]
   The quality or state of being rapid; swiftness; celerity;
   velocity; as, the rapidity of a current; rapidity of speech;
   rapidity of growth or improvement.
   [1913 Webster]

   Syn: Rapidness; haste; speed; celerity; velocity; swiftness;
        fleetness; quickness; agility.
        [1913 Webster]

WordNet (r) 3.0 (2006):

rapidity
    n 1: a rate that is rapid [syn: celerity, quickness,
         rapidity, rapidness, speediness]