Lewis & Short

răpĭdĭtas, ātis, f. [rapidus], swiftness of a stream that carries all before it; velocity, rapidity (only in the foll. passages): fluminis, Caes. B. C. 1, 62, 2; id. B. G. 4, 17, 2; Front. Strat. 1, 6, 2.