What is Saturated Output Power in an Amplifier?

Saturated output power is the maximum output power you can get out from an amplifier. There are a few stages to amplifier operation - the first one is linear operation i.e this is the stage where the output of the amplifier is equal to the input signal + gain. However, there comes a time, where the amplifier stops behaving linearly, i.e it does not amplify the input signal by the specified gain, the output of the amplifier starts to saturate and eventually the slope of the Input power vs Output power (as seen in the graph) of the amplifier becomes zero. The power level at which this happens is known as saturated output power.