https://doi.org/10.1351/goldbook.14804
An empirical relationship in which numerical parameters are associated with small perturbations in a parent molecule and are subsequently correlated with the change in the Gibbs energy of a certain reaction of the parent molecule vs. the perturbed molecules.
Note: A classical example is the well-known Hammett equation, in which sigma (\(\upsigma\)) values are constructed for specific substituents on a benzoic acid parent molecule. The linear correlation is then between the (summed) sigma values for a set of compounds and the \(\pu{p}K_{\rm{a}}\) (free energy of acid dissociation) of those compounds.