The extent to which quantum-mechanical spin is aligned in a particular direction. The concept applies to electrons, neutrons, and more generally any quantum-mechanical entity characterized by a spin. Spin polarization can be produced by an external magnetic field. In the case of electrons, spin polarization gives rise to spin-polarized currents. In the case of neutrons, it gives rise to polarized neutron scattering. Spin polarization is important in spintronics.