https://doi.org/10.1351/goldbook.A00079
  1. (of a compound) For @[email protected] the tendency of a compound to act as a @[email protected] donor. It can be quantitatively expressed by the acid @[email protected] constant of the compound in water or some other specified medium. For @[email protected] it relates to the @[email protected] constants of @[email protected] and @[email protected]
  2. (of a medium) The use of the term is mainly restricted to a medium containing @[email protected], where it means the tendency of the medium to hydronate a specific reference base. It is quantitatively expressed by the appropriate @[email protected]
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1081 [Terms] [Paper]