The negative of the @E02141@ change in the gas phase reaction (real or hypothetical) between a @P04906-1@ (more appropriately @H02904@) and the @CT01038@ concerned, usually an electrically neutral species to give the @C01266@ of that species. @P04906-2@ affinity is often, but unofficially, abbreviated as PA.
See also:
gas-phase basicity
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1153 [Terms] [Paper]