Entry 1 sense Webster, 1913 Coliseum /kälˌ-əs-ē'-əm/ · Col·i·se·um · IPA /[kʰɑɫəˈsi.əm]/ 01 n. The amphitheater of Vespasian at Rome, the largest in the world. 1. The amphitheater of Vespasian at Rome, the largest in the world.