storm sewer vs storm drain

storm sewer and storm drain both are nouns.

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

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

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

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

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