A polyatomic ion, also known as a molecular ion, is a charged species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix "poly-" means "many," in Greek, but even ions of two atoms are commonly referred to as polyatomic. In older literature, a polyatomic ion is also referred to as a radical, and less commonly, as a radical group. In contemporary usage, the term radical refers to free radicals that are (not necessarily charged) species with an unpaired electron.
For example, a hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is (OH)−. It has a charge of −1. An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: Its chemical formula is (NH4)+. It has charge of +1.
A polyatomic ion can often be considered as the conjugate acid or conjugate base of a neutral molecule. For example, the sulfate anion, SO42−, is derived from H2SO4, which can be regarded as SO3 + H2O.