velocity

in mass transport
https://doi.org/10.1351/goldbook.V06608
The
flux
density NB is related to the velocity by the equation NB=cB vB The vector vB is the macroscopic average velocity at which the species B moves (which is to be distinguished from the random molecular velocity); cB is the concentration of species B (molm3). Note that the velocity is defined with respect to a frame of reference.
Source:
PAC, 1981, 53, 1827. (Nomenclature for transport phenomena in electrolytic systems) on page 1830 [Terms] [Paper]