Intermodulation(IM) refers to undesired modulations that occur as sum frequencies or difference frequencies of two carrier signals.
Intermodulations occur at non-linear characteristics of active components, i.e. at diodes, transistors, field effect transistors( FET) and in electronic circuits and are formed from the mixed products of the individual harmonics. Depending on which harmonics sum up to an intermodulation frequency, one speaks of 2nd order or 3rd order intermodulation products.
The intermodulation frequencies formed from the sum and difference signals do not have an even-numbered relationship to the carrier frequency.