demesne vs realm

demesne and realm both are nouns.

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

As nouns, realm is a hyponym of demesne; that is, realm is a word with a more specific, narrower meaning than demesne:

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