amphitheatre vs stadium

amphitheatre and stadium both are nouns.

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

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

amphitheatre: 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 amphitheatre include arena, bowl, sports stadium.

amphitheatre (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)