Output Reflection Coefficent: G_out is the reflection coefient at the output terminals of the transistor looking into the transistor. The transistor input is connected to a source with a source impedance Z_s (or G_s).
G_outis given by
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 Output Impedance Z_out.