Noun
Abbe number (plural Abbe numbers)
A number that represents the reciprocal of the dispersive power of a substance with transparent qualities.
For optics in the visual range, the amount of dispersion of a lens material is often quantified by the Abbe number : : For a more accurate description of the wavelength dependence of the refractive index, the Sellmeier equation can be used. Source: Internet
In the visible part of the spectrum, diffractives have an Abbe number of −3.5. Source: Internet