threshold energy, E 0

https://dx.doi.org/10.1351/goldbook.T06366
Potential energy gap between reactants and the transition state, sometimes involving the zero point energies, but usually not.
See also: critical energy
Source:
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 189