Nuclear isomers are nuclear species with the same number of neutrons and the same number of protons, but different binding energy per nucleon are called isomers of that nuclide. Nuclear isomers shouldn't be confused with chemical isomers. These nuclei have organized the nucleons (protons and neutrons) into a slightly different energy configuration, one of which is almost always unstable. Usually, the protons or neutrons rearrange themselves and give off a gamma ray when giving off this excess energy. This is called gamma decay.
One well known and very useful nuclear isomer is technetium-99m. Tc-99m is the most used medical isotope for diagnostic procedures in the modern world. Tc-99m has a half life of 6 hours, while Tc-99 has a half life of 210,000 years. Both of these isomers have 43 protons and 56 neutrons, but Tc-99m doesn't last for nearly as long, and undergoes a decay.