The definition of an ion is an atom or molecule that has a net charge. This is the result of having different numbers of electrons and protons.
Another example is the hydroxide ion (
Oxygen has 6 valence electrons and hydrogen has one valence electron. It's not possible to create a Lewis structure for this ion and meet the octet rule for oxygen without adding one more electron. Adding one electron allows for a Lewis structure to meet the octet rule for oxygen and gives the ion a -1 charge. The ion has a single bond between the O-H and three non-bonding electron pairs around the oxygen.