Opera is a form of theatre in which music is a fundamental component and dramatic roles are taken by singers. Such a "work" (the literal translation of the Italian word "opera") is typically a collaboration between a composer and a librettist and incorporates a number of the performing arts, such as acting, scenery, costume, and sometimes dance or ballet. The performance is typically given in an opera house, accompanied by an orchestra or smaller musical ensemble, which since the early 19th century has been led by a conductor. Although musical theatre is closely related to opera, the two are considered to be distinct from one another.

Opera is a key part of Western classical music, and Italian tradition in particular. Originally understood as an entirely sung piece, in contrast to a play with songs, opera has come to include numerous genres, including some that include spoken dialogue such as Singspiel and Opéra comique. In traditional number opera, singers employ two styles of singing: recitative, a speech-inflected style, and self-contained arias. The 19th century saw the rise of the continuous music drama.

Opera originated in Italy at the end of the 16th century (with Jacopo Peri's mostly lost Dafne, produced in Florence in 1598) especially from works by Claudio Monteverdi, notably L'Orfeo, and soon spread through the rest of Europe: Heinrich Schütz in Germany, Jean-Baptiste Lully in France, and Henry Purcell in England all helped to establish their national traditions in the 17th century. In the 18th century, Italian opera continued to dominate most of Europe (except France), attracting foreign composers such as George Frideric Handel.