That value of pH in an aqueous phase at which the distribution ratio is unity at equilibrium. 50% of the solute is extracted (E = 0.5) only when the phase ratio is unity.
Source:
PAC, 1993, 65, 2373. 'Nomenclature for liquid-liquid distribution (solvent extraction) (IUPAC Recommendations 1993)' on page 2385 (https://doi.org/10.1351/pac199365112373)