1560s, also formerly Coliseum, Medieval Latin name for the classical Amphitheatrum Flavium (begun c. 70 C.E.), noun use of neuter of adjective colosseus "gigantic;" perhaps a reference to the colossal statue of Nero that long stood nearby (see colossus).
late 14c., "ancient Roman edifice for the exhibition of games, etc.," from Latin amphitheatrum, from Greek amphitheatron "double theater, amphitheater," neuter of amphitheatros "with spectators all around," from amphi "on both sides" (from PIE root *ambhi- "around") + theatron "theater" (see theater). Classical theaters were semi-circles, thus two together made an amphi-theater. They were used by the Romans especially for gladiatorial contests and combats of wild beasts.