A form of theatre in which the drama is conveyed wholly or predominantly through music and singing.

Opera emerged in Italy around 1600 and is generally associated with the Western classical music tradition. Opera uses many of the elements of spoken theatre such as scenery, costumes and acting, but is generally distinguished from other dramatic forms by the importance of song and conventions of vocal technique. The singers are accompanied by a musical ensemble ranging from a small instrumental ensemble to a full symphonic orchestra. Opera may also incorporate dance (this was especially true of French opera for much of its history).

Comparable art forms exist in various other parts of the world, many of them ancient in origin, and are also sometimes called 'opera' by analogy, usually prefaced with an adjective indicating the region (e.g. Chinese opera). These independent traditions are not derivative of Western opera, but are rather distinct forms of musical theatre.

Opera is also not the only type of Western musical theatre: in the ancient world, Greek drama featured singing and instrumental accompaniment, and in modern times, other forms such as the musical have appeared.