millwheel vs water wheel

millwheel and water wheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
millwheel	Yes	No	No	No
water wheel	Yes	No	No	No

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

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

Other hypernyms of millwheel include waterwheel.

millwheel (noun)	water wheel (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