The quality of moving or reacting with great speed
the fish sink into the sand with such rapidity that it must be seen to be believed
The fact of happening at a great rate; swiftness
technology spreads with extraordinary rapidity
celerity: a rate that is rapid
In relativity, rapidity is an alternative to speed as a framework for measuring motion. On parallel velocities (say, in one-dimensional space) rapidities are simply additive, unlike speeds at relativistic velocities. ...
speed, swiftness; the condition of being rapid; A measure of velocity relative to the speed of light; A measure of the velocity of a particle in a beam relative to the beam#s axis