The pH value at which the net electric charge of an elementary entity is zero. pI is a commonly used symbol for this kind-of-quantity. It should be replaced by pH(I) because it is a pH determined under that particular condition.
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 618 (https://doi.org/10.1351/pac197231040577)
PAC, 1994, 66, 891. 'Quantities and units for electrophoresis in the clinical laboratory (IUPAC Recommendations 1994)' on page 894 (https://doi.org/10.1351/pac199466040891)