water softener vs softener

water softener and softener both are nouns.

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

As nouns, softener is a hypernym of water softener; that is, softener is a word with a broader meaning than water softener:

water softener: a substance (such as sodium chloride) that lessens the hardness of water by replacing calcium and magnesium ions with sodium ions and so gives the water more efficient sudsing power
softener: a substance added to another to make it less hard

water softener (noun)	softener (noun)
a substance (such as sodium chloride) that lessens the hardness of water by replacing calcium and magnesium ions with sodium ions and so gives the water more efficient sudsing power	a substance added to another to make it less hard