water nymph vs undine

water nymph and undine both are nouns.

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

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

water nymph (noun)	undine (noun)
(Greek mythology) any nymph of the water	any of various female water spirits
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