main vs water main

main and water main both are nouns.

Word	Noun	Adjective	Verb	Adverb
main	Yes	Yes	No	No
water main	Yes	No	No	No

As nouns, water main is a hyponym of main; that is, water main is a word with a more specific, narrower meaning than main:

main: a principal pipe in a system that distributes water or gas or electricity or that collects sewage
water main: main (a pipe or conduit) for conveying water

Other hyponyms of main include electric main, gas main, sewer line, sewer main.

main (noun)	water main (noun)
a principal pipe in a system that distributes water or gas or electricity or that collects sewage	main (a pipe or conduit) for conveying water
any very large body of (salt) water

main (adjective)	water main (adjective)
(of a clause) capable of standing syntactically alone as a complete sentence
most important element
of force; of the greatest possible intensity