Binding energy is the energy required to split a whole system into multiple parts. At the atomic level the atomic binding energy of the atom derives from electromagnetic interaction and is the energy required to split the atom into loose electrons and a nucleus. Classically, a bound system has less mass than its unbound constituents. For systems with a high binding energy, the unbinding of a system results in a release of heat or light to account for the missing mass (due to Einstein's equation, $E = mc^2$.