For questions about ions - atomic or molecular particles having a net electric charge. Do NOT use this tag just because your question involves ions but is not about the ions themselves (as in electrochemistry, etc.).
Ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. An ion consisting of a single atom is an atomic or monatomic ion; if it consists of two or more atoms, it is a molecular or polyatomic ion.
The name was given by physicist Michael Faraday for the substances that allow a current to pass ("go") between electrodes in a solution when an electric field is applied. It is the transliteration of the Greek participle ἰόν, ión, "going".
According to the IUPAC goldbook:
An atomic or molecular particle having a net electric charge.