The existence of excited states of atomic nuclei with unusually long lifetimes. A nucleus may exist in an excited quantum state with well-defined excitation energy (E_x), spin (J), and parity (π). Such states are unstable and decay, usually by the emission of electromagnetic radiation (gamma rays), to lower excited states or to the ground state of the nucleus. The rate at which this decay takes place is characterized by a half-life (τ_1/2), the time in which one-half of a large number of nuclei, each in the same excited state, will have decayed. If the lifetime of a specific excited state is unusually long, compared with the lifetimes of other excited states in the same nucleus, the state is said to be isomeric. The definition of the boundary between isomeric and normal decays is arbitrary; thus the term is used loosely.