Meaning of counter-ion

coun•ter-i•on

Pronunciation: (koun'tur ī"un, -ī"on), [key]
— n. Physical Chem.
  1. an ion in solution that associates itself with an ion of opposite charge on the surface of a member of a solute.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.