Ion
Definition
An atom or molecule that carries an electric charge because it has gained or lost electrons.
Technical definition
An ion is an atom or molecule with a net electric charge resulting from the loss of electrons (a cation, positive) or the gain of electrons (an anion, negative). Ions are the charge carriers in electrolytes and the building blocks of ionic compounds.
Example
Table salt dissolves into Na+ and Cl- ions. The Na+ formed when a sodium atom lost one electron.
Common confusion
Gaining electrons makes a negative ion and losing electrons makes a positive one. The signs feel backwards to beginners, but losing negative charge leaves you positive: lose electrons = positive ion.