A macro-ion of a polyampholyte (in particular a protein) is said to be isoionic if besides the polyampholyte and
H+ or
OH− ions (in general ions of the solvent) no other ions are present in the system.
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 618 (https://doi.org/10.1351/pac197231040577)