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).
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 a vacuum, these are given by
where μ_{0} is the magnetic constant, or permeability of free space, equal to 4π × 10^{7} N·A^{2}, and c is the speed of light, 299,792,458 m/s.
In case of lossy medium (i.e. when the conduction currents are not negligible) the total current density flowing is:
where <math>j = \sqrt{1}</math>, 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:
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