amphitheater vs coliseum

amphitheater and coliseum both are nouns.

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

As nouns, amphitheater and coliseum are synonyms defined as:

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

Other synonyms of amphitheater include amphitheatre.

amphitheater (noun)	coliseum (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)