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 element. 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.
For more information on nuclear isomers please see nucleonica.