Permittivity

The permittivity $\varepsilon$ is a measure of the response of a dielectric material when subject to an electric field. Specifically, it is used to determined the dielectric polarization of the material. In the simplest case of linear dielectrics it is

where $\varepsilon_{0}$ is the permittivity of free space and $\chi_{e}$ is the electric susceptibility. The term permittivity of free space actually itself comes from this definition, as it is the permittivity when susceptibility is zero, that is, when there is no matter to polarize. This may lead to the idea that the vacuum is just a type of dielectric, but that is not the case. The behavior of the vacuum is complex and requires quantum field theory to appreciate completely.

As an aside, the value $1+\chi_{e}$ is also called the relative permittivity $\varepsilon_{r}$.