In an electromagnetic wave of a given frequency the strength of the electric field in volts per meter divided by the strength of the magnetic field in amps per meter.

Wave impedance is the ratio of the electric field strength to the magnetic field strength of an electromagnetic wave. The symbol Z is used to represent it and it's given in units of ohms, sybolized by the uppercase omega (Ω). The symbol η (eta) may be used instead of Z for wave impedance to avoid confusion with electrical impedance, although η is also the symbol for electromagnetic impedance, the light wave equivalent of wave impedance.