It can encode phase and frequency information when formulated as a complex number, appearing in that capacity in the Havrilak-Negami relaxation. For the purposes of analytical chemistry, it can be used as a measure of polarity, and when the material 
It can encode phase and frequency information when formulated as a complex number, appearing in that capacity in the Havrilak-Negami relaxation. For the purposes of analytical chemistry, it can be used as a measure of polarity, and when the material it describes is not isotropic, it is written as a second-rank tensor. If this value is negative for a material, along with the magnetic permeability, then optical effects result that include reversed Cerenkov radiation, an reversed Snell effect, and a reversed Doppler shift. For 10 points, name this quantity whose value for the vacuum is called the electric constant.
It can encode phase and frequency information when formulated as a complex number, appearing in that capacity in the Havrilak-Negami relaxation.
electric permittivity [prompt on permittivity]