TIN is the input reflection coefficient of a two port that is terminated with a terminating  impedance Zs on the signal side and a terminating impedance ZL on the output side. When this is done the input and output reflection coefficients are modified from s11 and s22. Here s11 and s22 are the reflection coefficients seen looking into the input and output of the two port ( or amplifier) before the terminating impedances are connected. TIN and TOUT ( addressed in another post) have a direct and significant effect on the design of a microwave amplifier. It is important therefore, to calculate these as part of the overall design.

Signal Processing Group Inc has published a calculator using javascript to allow a user to do these calculations quickly and efficiently. The calculator may be found on the SPG website under the complementary menu item.