velocity

in mass transport
https://doi.org/10.1351/goldbook.V06608
The @[email protected] density \(N_{\text{B}}\) is related to the velocity by the equation \[N_{\text{B}}=c_{\text{B}}\ v_{\text{B}}\] The vector \(v_{\text{B}}\) is the macroscopic average velocity at which the species B moves (which is to be distinguished from the random molecular velocity); \(c_{\text{B}}\) is the concentration of species B (\(\text{mol m}^{-3}\)). 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]