amphitheater vs sports stadium

amphitheater and sports stadium both are nouns.

Word	Noun	Adjective	Verb	Adverb
amphitheater	Yes	No	No	No
sports stadium	Yes	No	No	No

As nouns, sports stadium is a hypernym of amphitheater; that is, sports stadium is a word with a broader meaning than amphitheater:

amphitheater: an oval large stadium with tiers of seats; an arena in which contests and spectacles are held
sports stadium: a large structure for open-air sports or entertainments

Other hypernyms of amphitheater include arena, bowl, stadium.

amphitheater (noun)	sports stadium (noun)
an oval large stadium with tiers of seats; an arena in which contests and spectacles are held	a large structure for open-air sports or entertainments
a sloping gallery with seats for spectators (as in an operating room or theater)