Duomo is a term for a cathedral church. The formal word for a church that is currently a cathedral is cattedrale; a Duomo may be either a present or a former cathedral (the latter always in a town that no longer has a bishop nor therefore a cathedral, as for example Trevi). Some, like the Duomo of Monza, have never been cathedrals, although old and important. Many people refer to particular churches simply as "Il Duomo" or "The Duomo", without regard to the full proper name of the church. Similar words exist in other languages: Dom (German), Dóm (Hungarian & Slovakian), Dôme (French), Domkirke (Danish), Dómkirkja (Icelandic),Domo (Portuguese) Domkyrka (Swedish), Domkirke (Norwegian), Doms (Latvian), Toomkirik (Estonian), Tum (Polish), and Tuomiokirkko (Finnish). Also in these languages the respective terms do not necessarily refer to a church functioning as a cathedral, but also to proto-cathedrals or simply prominent church buildings, which have never been a cathedral in the exact sense of that word. In German the term Dom became the synecdoche, used - pars pro toto - for most existing or former collegiate churches. Therefore the uniform translation of these terms into English as cathedrals may not always be appropriate and should be used on a contextual basis.

According to the Oxford English Dictionary, and to Lo Zingarelli, the word duomo derives from the Latin word "domus", meaning house, as a cathedral is the "house of God", or domus Dei. The Garzanti online dictionary also gives the etymology as deriving from house, but house of the bishop ("domu(m) (episcopi); 'casa (del vescovo)') instead of the house of God.