A plasma with an electron density exceeding the critical density is called an over-dense plasma. In accord to the quasi-neutrality condition of a plasma, the plasma density equals the electron density. It is defined as
<math>n_c:=\varepsilon_o\,m_e\,\frac{\omega^2}{q_e^2}<math>
with
<math>\varepsilon_o<math>: dielectric constant of the vacuum
<math>m_e<math>: rest mass of the electron
<math>q_e<math>: charge of the electron
<math>\omega<math>: angular frequency of the electromagnetic field
