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An addition reaction joins reacting molecular entities while reducing bond multiplicity and retaining their atoms in the combined product. For example, adding hydrogen across an alkene's carbon–carbon double chemical bond produces a saturated product. The reverse transformation is an elimination reaction.^[ref]

The net transformation does not uniquely specify the mechanism. Electrophilic, radical and concerted routes can give addition products with different rate laws and stereochemistry. Tracking both the atoms and the changing bond order separates an addition reaction from a replacement or rearrangement.

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]

A substitution reaction replaces an atom or group in a molecular entity with another. The replacement can occur through one elementary step or several; the definition specifies the net chemical change rather than a unique mechanism.^[ref]

For example, in $R-X+N{u}^{-}\to R-Nu+X^{-}$, a nucleophile replaces a leaving group. Concerted displacement and a pathway through a carbocation can have different concentration dependences and stereochemical outcomes even when the written products match. The reacting functional group, solvent and competing elimination reactions help determine which pathway is observed.