is the electric susceptibility of the dielectric. Therefore,

.......................(2.76)

is called relative permeability or the dielectric constant of the medium. is called the absolute permittivity.

A dielectric medium is said to be linear when is independent of and the medium is homogeneous if is also independent of space coordinates. A linear homogeneous and isotropic medium is called a simple medium and for such medium the relative permittivity is a constant.

Dielectric constant may be a function of space coordinates. For anistropic materials, the dielectric constant is different in different directions of the electric field, D and E are related by a permittivity tensor which may be written as:

.......................(2.77)