From Academic Kids
A polyatomic ion is an ion consisting of a molecule with covalently bonded atoms or of a metal complex that can be considered as acting as a single unit in the context of acid/base chemistry or in the formation of salts. Note, a polyatomic ion is also referred to in older works as a radical. In current usage the term radical refers to free radicals which are uncharged species with an unpaired electron.
Note that many of the common negatively-charged anionic polyatomic ions are conjugate bases of acids derived from the oxides of non-metallic elements. For example the sulfate or SO42- ion is derived from H2SO4 which can be regarded as SO3 + H2O.