A nuclear isomer is a metastable state of an atom caused by the excitation of a proton or neutron in its nucleus so that it requires a change in spin before it can release its extra energy. Contrast this with the definition of a chemical isomer, the more common use of the word. Also contrast with the meaning of isotope, in which the difference is the number of neutrons in the nucleus. Metastable isomers of a particular atom are usually designated with an "m" (or, in the case of atoms with more than one isomer, 2m, 3m, and so on). This designation is usually placed after the atomic symbol and number of the atom (e.g., Co-58m), but is sometimes placed as a superscript before (e.g., mCo-58).
Most nuclear isomers are very unstable, and radiate away the extra energy immediately (on the order of 10-12 seconds). As a result, the term is usually restricted to mean isomers with half-lives of 10-9 seconds or more. Quantum mechanics predicts that certain atomic species will possess isomers with unusually long lifetimes even by this stricter standard, and so have interesting properties.
The only stable nuclear isomer is Ta-180m, which occurs naturally in tantalum at about 1 part in 8300. Its half-life is at least 1015 years, and it may in fact be entirely stable. The origin of this isomer is mysterious, though is it believed to have something to do with supernovas. When it relaxes to its base state, it releases energetic photons with wavelength of 16 nanometers -- x-ray wavelengths. There are reports that Ta-180m can be forced to release its energy by much weaker x-rays, but these are currently in scientific dispute.
Another reasonably stable nuclear isomer (with a half-life of 31 years) is hafnium-178m, which has the highest excitation energy of any stable isomer. One kilogram of pure Hf-178-2m contains approximately 900 gigajoules of energy, or about a quarter of a kiloton. Further, all of the energy released is in gamma rays at 0.05 nanometers. As with Ta-180m, there are disputed reports that Hf-178-2m can be stimulated into releasing its energy, and as a result the substance is being studied as a possible source for gamma ray lasers. These reports also indicate that the energy is released very quickly, so that Hf-178-2m can produce extremely high powers (on the order of exawatts).
Both of these isomers, as well as others, are currently being explored as a means of energy storage, as it is possible to "pump" standard atoms of the type into their higher states. However, all currently known methods of doing so are very inefficient, and knowledge of how to trigger a release of energy is still in its infancy.