An illicit discharge is defined as any discharge into a storm drain system that is not composed entirely of stormwater. Exceptions include water from firefighting activities and allowable discharges from facilities that are covered by an NPDES permit. Illicit discharges are a problem because, unlike municipal wastewater which flows to a treatment plant, stormwater generally flows to waterways without any additional treatment.
Illicit discharges often include pathogens, nutrients, surfactants, organic chemicals, and various toxic pollutants. They may be caused when a sewage disposal system interacts with the storm drain system, or when there are inadvertent or deliberate cross connections between industrial or commercial drains and the stormwater system. They may also occur when individuals dump contaminated liquids into storm drains, or when outdoor activities like pressure washing or radiator flushing cause polluted water to flow into the stormwater system or directly into a water body. Landscape irrigation can also produce intermittent illicit discharges if over-watering or misdirected sprinklers send water over impervious areas, producing unacceptable loads of nutrients, organic matter or pesticides.
This video from the Center for Watershed Protection describes Illicit Discharge Detection and Elimination (IDDE). MS4 operators are required to have an IDDE program as a condition of their permits.
More information on illicit discharges may be found here.