Relative permittivity is typically denoted as  (sometimes , lowercase kappa) and is defined as
where ε(ω) is the complex frequency-dependent permittivity of the material, and ε is the vacuum permittivity.
Relative permittivity is a dimensionless number that is in general complex-valued; its real and imaginary parts are denoted as:
The relative permittivity of a medium is related to its electric susceptibility, , as .
In anisotropic media (such as non cubic crystals) the relative permittivity is a second rank tensor.
The relative permittivity of a material for a frequency of zero is known as its static relative permittivity.