realm vs demesne

realm and demesne both are nouns.

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

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

Other hypernyms of realm include domain, land.

realm (noun)	demesne (noun)
a knowledge domain that you are interested in or are communicating about	territory over which rule or control is exercised
the domain ruled by a king or queen	extensive landed property (especially in the country) retained by the owner for his own use
a domain in which something is dominant