S-Parameters are typically used to characterize high frequency circuits and describe the transfer of power from one port to another. For a 2-port S-Parameter model: S11: Represents the input reflection coefficient (this is the most important parameter since it describes how much power is actually being reflected) S12 : Represents Isolation (how much power leaks from Port 2 to Port 1) S21: Represents Forward gain S22: Represents reflection at port 2