millwheel vs waterwheel

millwheel and waterwheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
millwheel	Yes	No	No	No
waterwheel	Yes	No	No	No

As nouns, waterwheel is a hypernym of millwheel; that is, waterwheel is a word with a broader meaning than millwheel:

millwheel: water wheel that is used to drive machinery in a mill
waterwheel: a wheel that rotates by direct action of water; a simple turbine

Other hypernyms of millwheel include water wheel.

millwheel (noun)	waterwheel (noun)
water wheel that is used to drive machinery in a mill	a wheel that rotates by direct action of water; a simple turbine
	a wheel with buckets attached to its rim; raises water from a stream or pond