The potential difference E D at zero electric current between two identical salt bridges, usually saturated
KCl bridges (conveniently measured by linking them to two identical electrodes) inserted into two solutions in Donnan equilibrium.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 619 (https://doi.org/10.1351/pac197231040577)