The arctangent of the ratio between two materials' values for this gives Brewster's Angle.  
The Abbe number is defined in terms of this quantity, which can be found using Cauchy's or Sellmeier's equation.  It is equal to the square root of the product of the relative permeability and the relative permittivity of the material, and metamaterials could have a negative value for this quantity.  The arctangent of the ratio between two materials' values for this gives Brewster's Angle.  It also appears in an equation that follows from Fermat's Principle, Snell's Law, and it is defined as the ratio of the speed of light in a vacuum to in a medium.  For 10 points, name this property which governs a namesake effect in which a beam of light is bent as it passes between mediums.
The Abbe number is defined in terms of this quantity, which can be found using Cauchy's or Sellmeier's equation.  
Index of Refraction [or Refractive Index]