duchy vs demesne

duchy and demesne both are nouns.

Word	Noun	Adjective	Verb	Adverb
duchy	Yes	No	No	No
demesne	Yes	No	No	No

As nouns, demesne is a hypernym of duchy; that is, demesne is a word with a broader meaning than duchy:

Other hypernyms of duchy include domain, land.

duchy (noun)	demesne (noun)
the domain controlled by a duke or duchess	territory over which rule or control is exercised
	extensive landed property (especially in the country) retained by the owner for his own use