In music, number refers to an individual song, dance, or instrumental piece which is part of a larger work of musical theatre, opera, or oratorio. It can also refer either to an individual song in a published collection or an individual song or dance in a performance of several unrelated musical pieces as in concerts and revues. Both meanings of the term have been used in American English since the second half of the 19th century.
Opera numbers may be arias, but also ensemble pieces, such as duets, trios, quartets, quintets, sextets or choruses. They may also be ballets and instrumental pieces, such as marches, sinfonias, or intermezzi. Until the mid-19th century most operas were structured as a series of discrete numbers connected by recitative or spoken dialogue. Oratorios followed a similar model. However, as the century progressed, numbers were increasingly unified into larger musical segments with no clear break between them. Early examples of this trend include Carl Maria von Weber's opera Euryanthe and Robert Schumann's secular oratorio Das Paradies und die Peri.