The negative of the
enthalpy change in the gas phase reaction (real or hypothetical) between a
proton (more appropriately
hydron) and the
chemical species
concerned, usually an electrically neutral species to give the
conjugate acid of that species.
Proton affinity is often, but unofficially, abbreviated as
PA.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1153