In left-handed materials, which have negative values of this quantity, the group and phase velocities point in opposite directions.  
The Sellmeier equation is an empirical relation describing the dependence of this quantity on wavelength.  In left-handed materials, which have negative values of this quantity, the group and phase velocities point in opposite directions.  Given by the square root of the product of the relative permittivity and permeability, for most materials it is approximately equal to the square root of the dielectric constant.  FTP, name this quantity, equal to the speed of light over the local phase velocity, which appears in Snell's Law.
The Sellmeier equation is an empirical relation describing the dependence of this quantity on wavelength.  
index of refraction (or refractive index; prompt on "n")