The energy needed to start a reaction.
This notion comes from chemistry: A certain minimum energy is needed to prime any reaction, which may then become self-maintained, or tend to come to a standstill, or run away as in an explosion process.
It would be interesting to explore analogies in ecology, economy and society. Many processes affecting systems in these domains seem to be triggered by some more or less precisely definite input of energy. However it also seems that these phenomena are very poorly understood and that in some cases disasters may result.
→ "chain reaction", "enthalpy", "runaway process".