Wikipedia - Acidity function (en) Wikipedia - Base não nucleofílica (pt) Wikipedia - تابع اسیدی (fa) Wikipedia - 酸度関数 (ja) acidity function

Also contains definitions of: basicity function, Hammett acidity function
https://doi.org/10.1351/goldbook.A00081
Any function that measures the thermodynamic @H02904@-donating or -accepting ability of a solvent system, or a closely related thermodynamic property, such as the tendency of the @L03653@ of the solvent system to form @L03510@. (The term 'basicity function' is not in common use in connection with basic solutions.) @A00079-1@ functions are not unique properties of the solvent system alone, but depend on the solute (or family of closely related solutes) with respect to which the thermodynamic tendency is measured. Commonly used @A00079-2@ functions refer to concentrated acidic or basic solutions. @A00079-2@ functions are usually established over a range of composition of such a system by UV/VIS spectrophotometric or NMR measurements of the degree of hydronation (protonation or @L03510@ formation) for the members of a series of structurally similar indicator bases (or acids) of different strength: the best known of these functions is the Hammett acidity function $$H_{0}$$ (for uncharged indicator bases that are primary aromatic @A00274@).
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1081 [Terms] [Paper]