What process combines two signals in a non-linear circuit to produce unwanted spurious outputs?
Why Any non-linear device (an overdriven amplifier, a corroded joint, a transistor biased into curvature) acting on two or more signals generates sums and differences of their frequencies and harmonics. When that happens where it was not intended, the resulting mix products are spurious emissions or spurious responses, and the process is called intermodulation. Third-order products at 2F1-F2 and 2F2-F1 are the troublesome ones because they land close to the original signals and fall inside the passband.
Watch out Heterodyning is the same physics used deliberately, a mixer combining a signal with a local oscillator to shift frequency; the pool reserves that term for the wanted result, while intermodulation names the unwanted one. Detection is recovering modulation from a carrier, and rolloff describes how a filter's response falls off past its cutoff.
Same physics, different intent: heterodyning is mixing you wanted, intermodulation is mixing you did not.