the difference in wavelength between the apex of the excitation spectrum (shorter wavelength, higher energy) and the apex of the emission spectrum (longer wavelength, lower energy).
The amount of difference between the excitation and emission color wavelengths given off by a fluorophore. The greater the difference, the less likely that the excitation light will interfere with the detection of the emission color (wavelength).
Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and luminescence spectra (or fluorescence) of the same electronic transition. It is named after Irish physicist George G. Stokes.