The interaction of the electron spin magnetic moment with the magnetic moment due
to the orbital motion of the electron. One consequence of spin-orbit
coupling is the mixing of zero-order states of different multiplicity. This effect may result
in fine structure called
spin-orbit splitting.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2277