mill wheel vs millwheel

mill wheel and millwheel both are nouns.

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

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

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

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