The
flux density

is related to the
velocity by the equation
The vector

is the macroscopic average velocity at which the species B moves (which is to be distinguished from the random molecular velocity);

is the concentration of species B
(

).
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
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
doi:10.1351/goldbook.