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 ion (positively charged) is the counterion for the chlorine ion (negatively charged) and vice versa. A counterion will be more commonly referred to as an anion or a cation, depending on whether it is negatively or positively charged. Thus, the counterion to an anion will be a cation, and vice-versa.
Any ion of the same charge as another in a solution or other electrochemical system.
Any ion of opposite charge to another in a solution or other electrochemical system