amphitheater vs stadium

amphitheater and stadium both are nouns.

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

As nouns, stadium is a hypernym of amphitheater; that is, 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
stadium: a large structure for open-air sports or entertainments

Other hypernyms of amphitheater include arena, bowl, sports stadium.

amphitheater (noun)	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)