A publically-owned conveyance or system of conveyances that discharges to waters of the U.S. and is designed or used for collecting or conveying storm water, is not a combined sewer, and is not part of a publicly-owned treatment works (POTW).
MUNICIPAL SEPARATE STORM SEWER SYSTEM (MS4)
Meaning of MUNICIPAL SEPARATE STORM SEWER SYSTEM (MS4) in English
Environmental engineering English vocabulary. Английский словарь экологического инжиниринга. 2012