principality vs demesne

principality and demesne both are nouns.

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

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

Other hypernyms of principality include domain, land.

principality (noun)	demesne (noun)
territory ruled by a prince	territory over which rule or control is exercised
	extensive landed property (especially in the country) retained by the owner for his own use