This expression is often applied to a reaction for which the overall standard Gibbs energy change ΔG° is negative. Some workers use this term with reference to a negative value of ΔH° at the absolute zero of temperature.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 165 (https://doi.org/10.1351/pac199668010149)