An essential ion is a subcategory of a cofactor. Essential ions are charged single atoms which are vital to the activation of an apoenzyme (an enzyme that is just sitting around, not interacting with anything) into a holoenzyme (a riproaring active enzyme that can react with substrates).

Essential ions can be further deconstructed into two subsubcategories: activator ions, which are loosely bonded to the enzyme and separate as soon as the enzyme is activated, and metalloenzymes, which are tightly bonded and stick to the enzyme throughout the whole process. Metalloenzymes are far more common than activator ions. For this reason, when speaking of essential ions, one usually is referring only to to metalloenzymes.