1. of a compound: For @B00744@ the tendency of a compound to act as a @H02904@ donor. It can be quantitatively expressed by the acid @D01801@ constant of the compound in water or some other specified medium. For @L03508@ it relates to the @A00472@ constants of @L03510@ and @A04388@.
  2. of a medium: The use of the term is mainly restricted to a medium containing @B00744@, where it means the tendency of the medium to hydronate a specific reference base. It is quantitatively expressed by the appropriate @A00081@
PAC, 1994, 66, 1077.(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1081 [Terms] [Paper]