In chemistry, an electrically charged molecule or atom is known as an ion because it has gained or lost electrons from its normal complement, a process known as ionization. Negatively charged ions are known as anions and positively charged ions are called cations.

The word "ion" is from Greek ion, present participle of ienai "go", thus "a goer". "Anion" and "cation" mean "up-goer" and "down-goer."

For single atoms in a vacuum, there are physical constants associated with the process of ionization. The energy needed to remove electrons from an atom is called the ionization energy, and the equivalent electrical potential -- i.e. the energy divided by a single electron charge -- is known as the ionization potential. These terms are also used to describe ionization of molecules and solids, but the values are not constant because ionization can be affected by local chemistry, geometry, and temperature.