The dielectric constant is a characteristic quantity of a given dielectric substance, sometimes called the relative permittivity (Bekefi and Barrett 1987, p. 418). In general, the dielectric constant is a complex constant, with the real part giving reflective surface properties (Fresnel reflection coefficients), and the imaginary part giving the radio absorption coefficient .
In cgs,

where is the electric susceptibility. In MKS,

where is the electric permittivity and is the permittivity of free space.
The corresponding quantity for magnetism is called the relative permeability .