spin projection

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 $S 2$ .

Source:

PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1963