The energy that an atomic system must acquire before a process (such as an emission or reaction) can occur WordNet 3.0 & Open English WordNet

In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. Wikipedia

Energy that must be input to a system to undergo a reaction or process Wikidata

The energy that must be overcome in order for a chemical reaction to occur. OmegaWiki

The energy required to initiate a reaction. For example, the flame from the fuse of a firecracker provides a small initial amount of energy, after which the explosive reaction proceeds by itself, releasing a considerably larger quantity of energy. A small push given to a stable but top-heavy object may cause it to fall over; the potential energy released during the fall was present in the system all along but could not be realized as long as the object was upright and balanced. Wiktionary

Activation energy. Wiktionary (translation)