water nymph vs fay

water nymph and fay both are nouns.

Word	Noun	Adjective	Verb	Adverb
water nymph	Yes	No	No	No
fay	Yes	No	No	No

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

Other hypernyms of water nymph include faerie, faery, fairy, sprite.

water nymph (noun)	fay (noun)
(Greek mythology) any nymph of the water	a small being, human in form, playful and having magical powers
a fairy that inhabits water
submerged aquatic plant having narrow leaves and small flowers; of fresh or brackish water
a water lily having large leaves and showy fragrant flowers that float on the water; of temperate and tropical regions