A multiplet, in spectroscopy, is a set of closely spaced, spectral lines showing fine structure. A multiplet arises from the splitting of a single line by, for example, electron spin or spin-orbit coupling.
In nuclear physics, a group of elementary particles, all hadrons with about the same mass, identical in all other properties except electric charge, usually having two or three members. The nucleons and pions form multiplet. In strong nuclear force interactions, members of a multiplet are all equivalent. A supermultiplet is a larger, more sophisticated and symmetrical grouping of hadrons involving eight quantum numbers, all of whose members have identical spin.