The difference (usually in frequency units) between the spectral positions of the
band maxima (or the band origin) of the absorption and
luminescence arising from the same electronic transition. Generally, the
luminescence occurring at a longer
wavelength than the absorption is stronger than the opposite. The latter may be called an anti-Stokes
shift.
Source:
PAC, 1996, 68, 2223
(Glossary of terms used in photochemistry (IUPAC Recommendations 1996))
on page 2278