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A molecular complex is an associated unit containing two or more component molecular entities, often held together by interactions weaker than ordinary covalent bonds. The components may be neutral or charged. The term needs context: coordination chemistry uses more specific descriptions for metal–ligand assemblies.^[ref]

A water dimer illustrates the idea: two water molecules associate through a hydrogen bond, while each retains its own ordinary O–H covalent bonds. Its overall composition is $({\mathrm{H}}_2\mathrm{O}{)}_2$. Distinguishing a bound pair from two independent molecules requires evidence about their association, rather than merely observing two water molecules in the same sample. Association and dissociation can coexist in an chemical equilibrium.

A molecular entity is an individually identifiable chemical unit, such as an atom, molecule, ion or molecular complex, distinguished by its composition and structure. The term is broader than molecule: a single sodium ion is a molecular entity even though it is not a neutral assembly of several atoms.^[ref]

One methane molecule, $\mathrm{C}{\mathrm{H}}_4$, is a molecular entity; a collection of chemically identical methane molecules is a chemical species. This distinction helps separate the individual objects in a reaction mechanism from the macroscopic amount of substance used in laboratory bookkeeping.^[ref]