The pressure difference which has to be established between a
colloidal system and its equilibrium liquid to prevent material transfer between the two phases when they are separated by a
membrane, permeable to all components of the system, except the
colloidal ones.
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 615