Wave impedance is a term used in acoustics, electromagnetics, and fluid dynamics to describe the relationship between the pressure (or electric field) and the particle velocity (or magnetic field) of a wave traveling through a medium. It serves as a measure of how much resistance a medium offers to wave propagation.
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For a plane electromagnetic wave in a homogeneous, linear, lossless, isotropic medium with permittivity and magnetic permeability , the wave impedance relates the field amplitudes by . Its units are ohms. Impedance differences determine the normal-incidence amplitude Fresnel equations, and , with a common tangential electric-field axis.