The hypothesis that each of several structural features of a
molecular entity makes a separate and
additive contribution to a property of the substance concerned. More specifically, it is the hypothesis that each of the several
substituent groups in a parent molecule makes a separate and
additive contribution to the standard Gibbs energy change (or
Gibbs energy of activation) corresponding to a particular equilibrium (or
rate of reaction).
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1081