Definition of ion

An atom or a group of atoms combined in a radical or molecule that carries a positive or a negative electric charge as a result of having lost or gained one or more electrons. It may exist in solution, usually in combination with molecules of the solvent, or out of solution; it may be formed during electrolysis and migrate to the electrode of opposite charge, or it may be formed in a gas and be capable of carrying an electric current through the gas.

Ref: Webster 3rd