In electromagnetism, the **absolute permittivity**, often simply called **permittivity** and denoted by the Greek letter *ε* (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in response to an applied electric field than a material with low permittivity, thereby storing more energy in the material. In electrostatics, the permittivity plays an important role in determining the capacitance of a capacitor.

In the simplest case, the electric displacement field **D** resulting from an applied electric field **E** is

More generally, the permittivity is a thermodynamic function of state ^{[1]}. It can depend on the frequency, magnitude, and direction of the applied field. The SI unit for permittivity is farad per meter (F/m).

The permittivity is often represented by the relative permittivity *ε*_{r} which is the ratio of the absolute permittivity *ε* and the vacuum permittivity *ε*_{0}

- .

This dimensionless quantity is also often and ambiguously referred to as the *permittivity*. Another common term encountered for both absolute and relative permittivity is the *dielectric constant* which has been deprecated in physics and engineering^{[2]} as well as in chemistry.^{[3]}

By definition, a perfect vacuum has a relative permittivity of exactly 1 whereas at STP, air has a relative permittivity of *κ*_{air} = 1.0006.

Relative permittivity is directly related to electric susceptibility (*χ*) by

otherwise written as

Moreover, the fact that the polarization can only depend on the electric field at previous times (i.e. effectively *χ*(Δ*t*) = 0 for Δ*t* < 0), a consequence of causality, imposes Kramers–Kronig constraints on the susceptibility *χ*(0).

As opposed to the response of a vacuum, the response of normal materials to external fields generally depends on the frequency of the field. This frequency dependence reflects the fact that a material's polarization does not change instantaneously when an electric field is applied. The response must always be *causal* (arising after the applied field), which can be represented by a phase difference. For this reason, permittivity is often treated as a complex function of the (angular) frequency ω of the applied field:

(since complex numbers allow specification of magnitude and phase). The definition of permittivity therefore becomes

*E*_{0}are the amplitudes of the displacement and electric fields, respectively,- i is the imaginary unit,
*i*^{2}= −1.The response of a medium to static electric fields is described by the low-frequency limit of permittivity, also called the static permittivity

*ε*_{s}(also*ε*_{DC}):- i is the imaginary unit,