A component, $M_S$, of the angular spin moment $S$ along an arbitrary axis (usually chosen as the z-direction). $M_S$ can take the values between $-S$ and $S$: $-S$, $-S+1$,..., $S-1$, $S$. The term is also used to denote an operation (spin-symmetry projection) allowing one to eliminate spin contamination in the cases where wavefunctions are not eigenfunctions of the operator $S^2$.