It is a point at which the power gain is down 1 dB from its ideal measure. It is a measure of nonlinearity for a device. See reference: "Gain Compression" slide of the Non-PLL RF Basics presentation       1.9 GHz and 100 MHz. See reference: "Ideal Mixers" slide of the Non-PLL RF Basics presentation           1.1 GHz and 800 MHz. See reference: "Interdemodulation Distortion" slide of the Non-PLL RF Basics presentation