in mass transport
The flux density N B is related to the velocity by the equation \[N_{\text{B}}=c_{\text{B}}\ v_{\text{B}}\] 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); cB 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 (