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.
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 (