in mass transport
The flux density N B is related to the velocity by the equation
N B = c B v B
The vector v B is the macroscopic average velocity at which the species B moves (which is to be distinguished from the random molecular velocity); c B is the concentration of species B (mol m −3). Note that the velocity is defined with respect to a frame of reference.
PAC, 1981, 53, 1827 (Nomenclature for transport phenomena in electrolytic systems) on page 1830