A wave that remains stationary, i.e. the displacement at any given point is always the same and a given displacement, such as that of a node, is not propagated along the wave. Standing waves result from the superimposition of two or more waves of the same period and usually occur when a wave is reflected totally or partially from a given barrier. Compare travelling wave.

In distributed circuits any mismatch of impedances will cause reflections of the voltage or current waves at the mismatch. The superposition of the incident and reflected voltage or current waves will result in a standing wave. If the amplitudes of the incident and reflected waves are the same then the standing wave will have a maximum amplitude of twice the amplitude of the individual waves; it will also possess nulls – points of zero amplitude – where the two waves exactly cancel. These points are located at every half wavelength of the original waves. In general, the incident and reflected waves will have different amplitudes, as some of the incident energy will be absorbed at the mismatched impedance. In this case the amplitude of the standing wave will be less than the maximum value, and the minimum will be nonzero, as the waves will no longer cancel exactly. In this situation the standing-wave ratio is defined as the ratio of maximum to minimum values of the standing wave voltage or current, and it is a measure of the impedance mismatch. The voltage standing-wave ratio (VSWR) is equal to

where Γ is the reflection coefficient of reflected to incident wave amplitudes due to the impedance mismatch.