An Etymological Dictionary of Astronomy and Astrophysics

The amount of change of some quantity during a time interval divided by the length of the time interval.

Etymology (EN): M.E. rate “monetary value,” M.Fr. rate “price, value,” from M.L. rata (pars) “fixed (amount),” from L. rata “fixed, settled,” p.p. of reri “to reckon, think,” → reason.

Etymology (PE): Nerx “rate, price, tariff.”