The signals from two commercial transmitters combine outside your receiver to produce noise on a desired frequency. What type of interference is this?

Intermodulation occurs when two or more strong signals interact within a non-linear device to produce new, unwanted frequencies. In this scenario, the signals are combining outside your receiver, such as at a corroded connection on a nearby tower or metal fence, which accidentally acts as a crude diode. Because the mixing happens externally before the combined signal even reaches your antenna, it is classified simply as intermodulation (or external intermodulation). It is not receiver overload, because the receiver's internal circuits are not the cause of the mixing. It is also not a harmonic or spurious emission, as those originate from a single transmitter's imperfections rather than the mathematical combination of multiple different transmitters.