When two or more radio frequency (RF) signal mix. The mixing can produce a summed, difference, or harmonic multiple frequency. The more wireless systems being used in an area, the higher the chance is that there will be IM trouble.
In a receiver, an unwanted signal sometimes interacts with the desired signal. The desired signal appears to be modulated by the undesired signal
Interference caused by mixing of signals from more than one transmitter. The mixing occurs when the signals are rectified by something that can act as a diode. Diode mixing is part of the process used to receive signals, but can also occur "accidentally" inside or outside of the receiver itself as the signal encounters a rusty metal joint or similar object. It is often referred to as "intermod".