An illicit discharge is defined in the federal regulations as:
"...any discharge to an Municipal Separate Storm Sewer Systems (MS4) that is not composed entirely of storm water..."
There are two modes of entry: direct and indirect.
Flows in the storm drains created by illicit/improper connections between sanitary sewer and storm drain system.
Flows in the storm drains created by pollutants that enter the storm drain system through an inlet or through the joints of the pipe (infiltration).