This is usually taken to be a reaction for which the overall standard Gibbs energy change \(\Delta G^{\,\unicode{x26ac}}\) is positive. Some workers use this term with respect to a positive value of \(\Delta H^{\,\unicode{x26ac}}\) at the absolute zero of temperature.
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 164 [Terms] [Paper]