realm vs knowledge domain

realm and knowledge domain both are nouns.

Word	Noun	Adjective	Verb	Adverb
realm	Yes	No	No	No
knowledge domain	Yes	No	No	No

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

realm: a knowledge domain that you are interested in or are communicating about
knowledge domain: the content of a particular field of knowledge

Other hypernyms of realm include domain, knowledge base.

realm (noun)	knowledge domain (noun)
a knowledge domain that you are interested in or are communicating about	the content of a particular field of knowledge
the domain ruled by a king or queen
a domain in which something is dominant