Third-order intercept point

In telecommunication, a third-order intercept point is a point (a) that is an extrapolated convergence--not directly measurable--of intermodulation distortion products in the desired output and (b) that indicates how well a receiver performs in the presence of strong nearby signals.

Note: Determination of a third-order intercept point is accomplished by using two test frequencies that fall within the first intermediate frequency mixer passband. Usually, the test frequencies are about 20 to 30 kHz apart.

Source: From Federal Standard 1037C and from MIL-STD-188