An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge. The word ion is the transliteration of the Greek participle ἰόν (meaning "going"), and was first used by physicist Michael Faraday for the substances that move ("go") under a current between electrodes in a solution, when an electric field is applied.
If a neutral atom loses one or more electrons, it has a net positive charge and is known as a cation. If an atom gains electrons, it has a net negative charge and is known as an anion. An ion consisting of a single atom is an atomic or mono atomic ion; if it consists of two or more atoms, it is a molecular or poly atomic ion.
In the case of ionization of a gas, what are known as "ion pairs" are created consisting of a free electron and a positive ion.
Read more about Ion: Anions and Cations