The pressure difference Δ p across the membrane, plug, etc., needed just to stop electro-osmotic volume flow. Δ p is positive if the higher pressure is on the high potential side.
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 620 (https://doi.org/10.1351/pac197231040577)