https://doi.org/10.1351/goldbook.E02279

This term applies in a general way to any multiparametric extension of the Hammett equation. It is sometimes used specifically for a form of dual substituent-parameter equation in which the actual value of the correlated property P under the influence of the substituent X is used, rather than the value relative to that for X = H. An intercept term h corresponding to the value of P for X = H is introduced, e.g. \[P=\alpha \ \sigma _{I}+\beta \ \sigma _{R}+h\] The equation may be applied to systems for which the inclusion of further terms to represent other effects, e.g. steric, is appropriate.*Source: *

PAC, 1994,*66*, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1114 (https://doi.org/10.1351/pac199466051077)

PAC, 1994,