Proportionality constant D, relating the flux of amount of entities B
to their concentration gradient Jn = -D.∇.cB.
Green Book, 2nd ed., p. 65 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 969 (https://doi.org/10.1351/pac199668040957)