https://doi.org/10.1351/goldbook.D01866

In a general sense, any equation which expresses substituent effects in terms of two parameters. However, in practice the term is used more specifically for an equation for summarizing the effects of *meta*- or *para*- substituents (i = m or p) **X** on chemical reactivity, spectroscopic properties, etc. of a probe site **Y** in benzene or other aromatic system. \[P^{\textrm{i}}=\rho _{I}^{\textrm{i}}\ \sigma _{I}+\rho _{R}^{\textrm{i}}\ \sigma _{R}\] P is the magnitude of the property **Y** for substituent **X**, expressed relative to the property for X = H; σ I and σ R are inductive or polar and resonance substituent constants, respectively, there being various scales for σ R and ρ R are the corresponding regression coefficients.*See also:* extended Hammett equation*Source: *

PAC, 1994,*66*, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1107 (https://doi.org/10.1351/pac199466051077)

PAC, 1994,