Question

What is proton affinity?

Answer 1

these reactions are always exergonic in the gas phase.
Proton affinity of an union or of a neutral atom or molecule is a measure of its gas phase basicity.

Answer 2

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.

enthalpy means change in heat. 
gas phase reaction  means chemical reactions between gaseous materials are quite similar to reactions between solids and liquids.