multiplicity (spin multiplicity)

The number of possible orientations, calculated as $2 S + 1$ , of the spin angular momentum corresponding to a given total spin quantum number ( $S$ ), for the same spatial electronic wavefunction. A state of singlet multiplicity has $S = 0$ and $2 S + 1 = 1$ . A doublet state has $S = 1 2$ , $2 S + 1 = 2$ , etc. Note that when $S > L$ (the total orbital angular momentum possible) there are only $2 L + 1$ orientations of total angular momentum possible.

Source:

PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2255