1. One negatively and one positively charged ion formed by the transfer of an electron from one neutral atom or molecule to another one.
2. A positive ion and an equal-charge negative ion, usually an electron, that are produced by the action of radiation on a neutral atom or molecule.
3. A pair of ions of equal and opposite charge formed by photoionization or by the interaction of matter with any sufficiently energetic particles; such as, beta particles or alpha particles.

An ion pair, in the context of chemistry, consists of a positive ion and a negative ion temporarily bonded together by the electrostatic force of attraction between them.

Ion pairs occur in concentrated solutions of electrolytes (substances that conduct electricity when dissolved or molten).

Thus, in concentrated solutions of sodium chloride, some positive sodium ions, Na+, and some negative chloride ions, Cl-, are paired together.

Upon colliding, two oppositely charged ions stay together only for a short period of time. On the average, a certain population of these pairs exists at any given time, although the formation and dissociation of ion-pairs is continuous.