millwheel vs mill wheel

millwheel and mill wheel both are nouns.

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

As nouns, millwheel and mill wheel are synonyms defined as:

millwheel and mill wheel: water wheel that is used to drive machinery in a mill

millwheel (noun)	mill wheel (noun)
water wheel that is used to drive machinery in a mill	water wheel that is used to drive machinery in a mill