Scattering matrix in microwave amplifier
Scattering parameters (S parameters) are a way of describing the behavior of a multi-ported circuit at a single frequency. In general, S parameters will tell you what happens when RF energy in the form of a sine wave is injected into any combination of ports - how much is reflected and with what phase angle, and how much makes it through to each of the other ports. It's really used for just about every RF component (filters, splitters/combiners, etc.), not just amplifiers.
You can use scattering parameters to reason about device performance (gain, isolation, etc.) and what happens when multiple devices are connected together.