http://www.microwaves101.com/encyclopedia/compressionpoint.cfm
What's a compression point?
For a linear device, output power is merely a fixed fraction of the input power. This includes most passive devices such as connectors, cable, waveguides, etc. Nonlinear devices exhibit complex behavior when input power is compared to output power. However, most nonlinear devices tend to become lossier with increasing input power. Devices such as amplifiers, mixers and switches tend to fall into this category.
At some power level, the gain response of the device will become reduced by a specific amount. This power level is said to be the compression point. Quite often, microwave engineers refer to the one-dB compression point, but two or three dB compression points are often important in power amplifier chains. Thus we refer to the quantities P1dB, P2dB, P3dB. In switches and other passives, you might hear reference to 0.25 dB compression point.
One general point about P1dB measurements that you should know: if you are working on a transmitter or exciter (or power amplifier), it is customary to specify the P1dB point at the output (abbreviated P1dBout). For a receiver (or low noise amp), the input P1dB point is often specified (abbreviated P1dBin). That being said, low noise amplifiers, the most critical component in a receiver chain, are most often specified with P1dB data measured at their output. If you are the receiver guy, you will need to calculate the worst-case P1dB at the input using the range of available gain that the part may provide.
Why do we talk about "gain" in a passive device such as a switch or a mixer, when obviously they have no gain? Because chances are components will become part of a larger chain, which will include amplification, and we want to be consistent.
