- 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@.
- 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@