hart vs American elk

hart and American elk both are nouns.

Word	Noun	Adjective	Verb	Adverb
hart	Yes	No	No	No
American elk	Yes	No	No	No

As nouns, American elk is a hypernym of hart; that is, American elk is a word with a broader meaning than hart:

Other hypernyms of hart include Cervus elaphus, elk, red deer, wapiti.

hart (noun)	American elk (noun)
a male deer, especially an adult male red deer	large North American deer with large much-branched antlers in the male
	common deer of temperate Europe and Asia