Representation theory
In representation theory, a multiplet refers to a representation of a mathematical structure.
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Quantum physics
In quantum physics, the mathematical notion is usually applied to the mathematical structure of gauge algebras. Quantization provides a correspondence between symmetries of gauged structures and particles. Thus, a multiplet has also come to describe the subatomic particles described by these representations.
Multiplet may also describe a group of related spectral lines.
Examples
The best known example is a spin multiplet, which describes symmetries of a group representation of an SU(2) subgroup of the Lorentz algebra, which is used to define spin quantization. A spin singlet is a trivial representation, a spin doublet is a fundamental representation and a spin triplet is a vector representation.
In QCD, quarks are in a multiplet of SU(3).
Seismology
In seismology, multiplet refers to a repeating earthquake, occurring in nearly the same location, with nearly the same source characteristics.