driving force (affinity) of a reaction, A

The decrease in Gibbs energy on going from the reactants to the products of a chemical reaction (− Δ G).
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1107
See also:
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2237