coliseum vs amphitheater

coliseum and amphitheater both are nouns.

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

As nouns, coliseum and amphitheater are synonyms defined as:

coliseum and amphitheater: an oval large stadium with tiers of seats; an arena in which contests and spectacles are held

Other synonyms of coliseum include amphitheatre.

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