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]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "rapidity":
   air speed, alacrity, bat, briskness, ceaselessness, celerity,
   chattering, constancy, constant flow, continualness, continuity,
   decisiveness, dispatch, expedition, expeditiousness, fastness,
   flight, flit, flurry, gait, ground speed, haste, hurry, immediacy,
   immediateness, incessancy, instantaneousness, knots,
   lightning speed, miles per hour, noninterruption, oscillation,
   pace, perpetuity, precipitation, promptitude, promptness,
   pulsation, punctuality, punctualness, quick fire, quickness,
   rapid fire, rapid recurrence, rapid succession, readiness,
   regularity, repetition, round pace, rpm, rush, snappiness, speed,
   speediness, staccato, steadiness, stuttering, summariness,
   sustainment, swift rate, swiftness, tattoo, unintermission,
   uninterruption, velocity, vibration