Input Impedance: Z_in is the input impedance at the input terminals of the transistor looking into the transistor. The transistor output is connected to a load with a load impedance Z_L (or G_L).
This reflection coefficent is usually a complex quantity. In the actual design of the amplifier, it plays a major role in determining the overall gain of the amplifier. It is used interchangable with the Input Reflection Coefficient G_in.