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Noun. 1. cation-a positively charged ion. hydrogen ion-a positively charged atom of hydrogen; that is to say, a normal hydrogen atomic nucleus. ion-a particle that is electrically charged (positive or negative); an atom or molecule or group that has lost or gained one or more electrons.
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. In a charged transition metal complex, a (i.e. non-coordinated) ion accompanying the complex is termed the counterion. In the older literature, the term gegenion (from gegen which means counter in German) is sometimes used.