intrinsic activation energy, E a,i

https://dx.doi.org/10.1351/goldbook.I03134
If E a,1 and E a,−1 are the activation energies for a reaction in forward and reverse directions, the lesser of the two is sometimes called the intrinsic activation energy. It is the activation energy for the reaction in the exothermic direction.
Source:
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 170