A polyatomic ion is an ion consisting of multiple atoms associated together by covalent bonds which can be considered as acting as a single unit in the context of acid/base chemistry or in the formation of salts. Note that in this context, a polyatomic ion is also referred to as a radical.

Some commonly-occurring polyatomic ions and their charge ub>2-

bicarbonate  HCO3-

nitrate      NO3-

phosphate    PO43-

sulfate      SO42-

acetate      CH3C(=O)O-c2h3o2

ammonium     NH4+

hydroxide    OH-

pyridinium   C5H5NH+
The atoms of a polyatomic ion are always associated with each other via covalent or coordinate-covalent bondss.

A polyatomic ion that is primarily associated with other atoms by covalent, rather than ionic, forces might more properly be referred to as a functional group.

Note that many of the common negatively-charged (anionic) polyatomic ions are oxides of non-metallic elements, and can be considered the conjugate bases of the corresponding acids.