The energy necessary to separate a combined system.
In particular the energy to separate a molecular binding.
Atoms combine to a chemical binding (molecules) because they find a lower energy level in the molecular binding than in the atomic stadium. The reason is, that valence electrons of one atom which are on a not completed maximum energy level find spare positions on a lower energy level of another atom. In consequence the electrons of both atoms togeteher are on a lower resulting energy level than in the separated atoms. The difference of energy is the binding energy. This amount of energy needs to be induced if a chemical binding shall be broken to separate the atoms.
The physical unit used for the binding energy is [kJ/mol].
Sufficient energy to separate chemical bindings can be induced to the molecule by very high temperature or by high energy radiation, so called ionizing radiation which can be high frequency UV-radiation as it is generated by low pressure plasma. When molecular bindings are separate by high energy stimulation, they are tranformed to radicals.