What are S-Parameters?

S-parameters (or scattering parameters) are used to describe how energy can propagate through an electric network. S-Parameters are used to describe the relationship between different ports, when it becomes especially important to describe a network in terms of amplitude and phase versus frequencies, rather than voltages and currents. S-Parameters are used to show a complicated network as a simple black box, and to easily present what happens to the signal in that network.

The S-parameters can be saved e.g. as a S4P-file that contains all the combinations of the reflection and transmissions in a network, and this shows how the device under test behaves with a signal in both forward and reverse directions.