parasitic emission
In a communications system in which one or more electromagnetic sources are used, electromagnetic radiation–such as lightwaves, radio waves, microwaves, X-rays, or gamma rays from one or more of the sources–that is not harmonically related, i.e., is not coherent, with the transmitted carrier.
Note: Parasitic emissions are usually caused by undesired oscillations or energy-level transitions in the sources. [From Weik ’89]