(n.) The quality or state of being rapid; swiftness; celerity; velocity; as, the rapidity of a current; rapidity of speech; rapidity of growth or improvement.

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