Definition:
Direct discharge involves the discharge of pollutants to waters of the US.  Discharge of a pollutant means: (a) Any addition of any ‘‘pollutant’’ or combination of pollutants to ‘‘waters of the United States’’ from any ‘‘point source,’’ or (b) Any addition of any pollutant or combination of pollutants to the waters of the ‘‘contiguous zone’’ or the ocean from any point source other than a vessel or other floating craft which is being used as a means of transportation. This definition includes additions of pollutants into waters of the United States from: surface runoff which is collected or channelled by man; discharges through pipes, sewers, or other conveyances owned by a State, municipality, or other person which do not lead to a treatment works; and discharges through pipes, sewers, or other conveyances, leading into privately owned treatment works.  This term does not include an addition of pollutants by any ‘‘indirect discharger.’’

Source:
40 CFR 122