  Over one hundred years have passed since the nuclear isomer was first
introduced, in analogy with chemical isomers to describe long-lived excited
nuclear states. In 1921, Otto Hahn discovered the first nuclear isomer
$^{234m}$Pa. After that, step by step, it was realized that different types of
nuclear isomers exist, including spin isomer, K isomer, seniority isomers, and
``shape and fission'' isomer. The spin isomer occurs when the spin change
$\Delta I$ of a transition is very large. The larger $\Delta I$, the lower the
electromagnetic transition rates, the longer the half-lives. The K-isomer
exists due to the significant change in K, where K is the projection of the
total angular momentum on the symmetry axis. The seniority isomers arise due to
a very small transition probability in seniority conserving transitions around
semi-magic nuclei, where the seniority, which corresponds to the number of
unpaired nucleons, is a reasonably pure quantum number. For a so-called shape
isomer, the inhibition of the decay transition comes from the associated shape
changes. It is caused by that a nucleus is trapped in a deformed shape which is
its secondary minimum and is hard to decay back to its ground state.
