bison vs American bison

bison and American bison both are nouns.

Word	Noun	Adjective	Verb	Adverb
bison	Yes	No	No	No
American bison	Yes	No	No	No

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

bison: any of several large humped bovids having shaggy manes and large heads and short horns
American bison: large shaggy-haired brown bison of North American plains

Other hyponyms of bison include American buffalo, Bison bison, buffalo, aurochs, Bison bonasus, wisent.

bison (noun)	American bison (noun)
any of several large humped bovids having shaggy manes and large heads and short horns	large shaggy-haired brown bison of North American plains