diffusion coefficient, D

https://doi.org/10.1351/goldbook.D01719
Proportionality constant D, relating the flux of amount of entities B to their concentration gradient Jn = -D.∇.cB.
Source:
Green Book, 2nd ed., p. 65 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)