art historian vs historian

art historian and historian both are nouns.

Word	Noun	Adjective	Verb	Adverb
art historian	Yes	No	No	No
historian	Yes	No	No	No

As nouns, historian is a hypernym of art historian; that is, historian is a word with a broader meaning than art historian:

art historian: a historian of art
historian: a person who is an authority on history and who studies it and writes about it

Other hypernyms of art historian include historiographer.

art historian (noun)	historian (noun)
a historian of art	a person who is an authority on history and who studies it and writes about it