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.
is the magnitude of the property **Y** for substituent **X**, expressed relative to the property for
X
= H;
and are inductive or polar and resonance substituent constants, respectively, there being various scales for ; and 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