What are multiple atoms that are covalently bonded and act as a single unit called?

Multiple atoms that are covalently bonded together and operate as a single unit have a specific classification known as polyatomic ions. These ions consist of two or more atoms held together by covalent bonds, which collectively carry a positive or negative charge.

For example, the sulfate ion (SO₄²⁻) is a polyatomic ion comprising one sulfur atom covalently bonded to four oxygen atoms, and it functions as a single charged unit in chemical reactions. This is distinct from molecular compounds, which involve molecules made up of covalently bonded atoms, and are not necessarily charged. Complex ions also involve a metal atom surrounded by ligands but do not fit the same definition. Binary compounds refer to a type of compound consisting of two different elements, which does not specifically cover the concept of covalently bonded groups acting as one. Thus, the most precise term for a group of covalently bonded atoms functioning collectively is indeed a polyatomic ion.