A term referring to the fact that two or more stationary states of the same quantum-mechanical system may have the same energy even though their wave functions are not the same. In this case the common energy level of the stationary states is degenerate. The statistical weight of the level is proportional to the order of degeneracy, that is, to the number of states with the same energy; this number is predicted from Schrödinger's equation. The energy levels of isolated systems (that is, systems with no external fields present) comprising an odd number of so-called fermions (for example, electrons, protons, and neutrons) always are at least twofold degenerate.