A macro-ion of a @[email protected] (in particular a protein) is said to be isoionic if besides the @[email protected] 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 [Terms] [Paper]