What is the energy level necessary to enable a reaction to occur called?
Activation energy (energy required to enable a reaction to 'start' rather than 'occur')
It effectively represents the energy needed to break the bonds in the reactant species and allow the reaction to go ahead. Once it has been supplied at the start, the energy released by the reaction acts as its own activation energy, and you don't need to keep applying it.
If the activation energy is high, the reaction will be kinetically stable and not go off spontaneously, even if it is a highly exothermic reaction (one which gives off a lot of heat).
If it is low, the reaction will start very easily (will often be spontaneous) - we say the reaction is kinetically unstable.
Activation energy can be represented as a 'hump' in energy diagrams for reactions.