Intersection of 2nd and 3rd order intermod products... Cool name... I used to specify amps and microwave components. Always looked for that value. One trick manufacturers used to embellish the value was not saying input or output IP..... it makes a difference.... :0)
I designed them and built them. Masers, tunnel diode amplifiers, low noise and power bipolar and FET transistor amplifiers and a few other microwave gadgets.
And along the way I discovered a simple method for measuring the IP of microwave amplifiers in less than a minute once you had a setup with two sources and a spectrum analyzer.
The good old days.