From AMS Glossary
Any of the possible discrete energies of an atom, molecule, or nucleus.
According to Newtonian mechanics, the energy of a body is a continuous variable, but according to the quantum theory, measured energies of bound states are discrete, some said to be allowed, others forbidden. We are not aware of this discreteness for macroscopic objects because of the extreme (relative) smallness of the separation between their energy levels. During transitions from one energy level to another, quanta of radiant energy are emitted or absorbed, their frequency depending on the difference between energy levels. Emissions of this type are responsible for the aurora borealis, for example.