drain system vs storm sewer

drain system and storm sewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
drain system	Yes	No	No	No
storm sewer	Yes	No	No	No

As nouns, drain system and storm sewer are synonyms defined as:

drain system and storm sewer: a system of watercourses or drains for carrying off excess water

Other synonyms of drain system include drainage well, storm drain, stormwater drain.

drain system (noun)	storm sewer (noun)
a system of watercourses or drains for carrying off excess water	a system of watercourses or drains for carrying off excess water