What is "counterion"

n. (context chemistry English) Any ion of opposite charge to another in a solution or other electrochemical system

thumb|160px|right| Polystyrene sulfonate, a cation exchange resin, is typically supplied with Na as the counterion. A counterion (pronounced as two words, i.e. "counter" "ion", and sometimes written as two words) is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt (NaCl), the sodium cation is the counterion for the chlorine anion and vice versa.