Output Impedance: Z_out is the output impedance 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).
This impedance 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 Output Reflection Coefficient G_out.