A chemical reaction cannot occur, except by quantum-mechanical tunnelling unless the total energy available is greater than a certain energy, which is known as the critical energy, or the threshold energy.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 162 (https://doi.org/10.1351/pac199668010149)