In electromagnetism,
the **permittivity** ε of a medium is
the ratio **D** / **E** where **D** is the electric displacement in coulombs per square metre (C/m^{2}) and **E** is the electric field strength in volts per metre (V/m). In the common case of an *isotropic* medium, **D** and **E** are parallel and ε is a scalar, but in more general *anisotropic* media this is not the case and ε is a rank-2 tensor (causing birefringence).

Permittivity is specified in farads per metre (F/m).
It can also be defined as a dimensionless **relative permittivity**, or **dielectric constant**, normalized to the absolute vacuum **permittivity** ε_{0} = 8.854 10^{-12}F/m.

When an electric field is applied, a current flows. The **total current** flowing in a real medium is in general made of two parts: a conduction current and a displacement one. A **perfect dielectric** is a material that shows displacement current only.

The permittivity ε and magnetic permeability μ of a medium together determine the velocity of electromagnetic radiation through that medium.

In case of lossy medium (i.e. when the conduction currents are not negligible) the total current density flowing is:

where , **σ** is the conductivity (responsible for conduction current) of the medium and **ε _{d}** is the relative permittivity (responsible for displacement current).

In this formalism the **complex permittivity ε ^{*}** is defined as:

At a given frequency, the imaginary part of ε leads to absorption loss if it is negative (in the above sign convention for frequency) and gain if it is positive. (More generally, one looks at the imaginary parts of the eigenvalues of the anisotropic dielectric tensor.)