In Spectroscopy you may easily think for Dispersion of how “wide” the whole spectrum in wavelength is spread. But dispersion is not able to define how well the final Spectrometer can separate two very near bands of a Spectrum; that’s called “Spectral Resolution”, because an additional parameter is needed, the so called “Band half width” (SBW). The SBW strongly, but not lonely!, depends on the Entrance AND the Exit Slit of the Monochromator.
The SBW (“Spectral Band Width”) is defined as the width (in nm), at half the maximum intensity of the band of Light leaving the monochromator.
As you are not able to use Slit width equal zero (otherwise you have nothing left to measure!), SBW will be "less" (higher) then the dispersion would allow it to be!
Dispersion (for Prisms) depends on the material and Diffraction (for Gratings) depends on the number of grooves or rulings per millimetre and a few other parameters.
“Spectral Resolution” is measured as the ratio of the valley between both Benzene or both Toluene Peaks around 260 till 270 nm in Hexane. The value should be as high (the valley as deep) as possible.
“Spectral Resolution” is an important parameter to qualify Spectrometers.