In a considerable number of cases plots of

vs.

,
for a series of reactions, e.g. for a reaction in a range of different solvents, are
straight lines of approximately unit slope. Therefore, the terms

and

in the expression partially compensate, and
often is a much simpler function of solvent (or other) variation than

or

separately.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1098
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.