American elk vs hart

American elk and hart both are nouns.

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

As nouns, hart is a hyponym of American elk; that is, hart is a word with a more specific, narrower meaning than American elk:

Other hyponyms of American elk include stag, hind, brocket.

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