In music, an intermezzo (, Italian pronunciation: [interˈmɛddzo], plural form: intermezzi), in the most general sense, is a composition which fits between other musical or dramatic entities, such as acts of a play or movements of a larger musical work. In music history, the term has had several different usages, which fit into two general categories: the opera intermezzo and the instrumental intermezzo. Renaissance intermezzo
The Renaissance intermezzo was also called the intermedio. It was a masque-like dramatic piece with music, which was performed between the acts of a play at Italian court festivities on special occasions, especially weddings. By the late 16th century, the intermezzo had become the most spectacular form of dramatic performance, and an important precursor to opera. The most famous examples were created for Medici weddings in 1539, 1565, and 1589. In Baroque Spain the equivalent entremés or paso was a one-act comic scene, often ending in music and dance, between jornadas (acts) of a play. Opera intermezzo
The intermezzo, in the 18th century, was a comic operatic interlude inserted between acts or scenes of an opera seria. These intermezzi could be substantial and complete works themselves, though they were shorter than the opera seria which enclosed them; typically they provided comic relief and dramatic contrast to the tone of the bigger opera around them, and often they used one or more of the stock characters from the opera or from the commedia dellarte.
Shostakovich named one movement of his dark String Quartet No. 15 "intermezzo"; Bartók used the term for the fourth movement (of five) of his Concerto for Orchestra. See also
Entracte
Divertimento
Interlude (disambiguation)
Entremet
Entremés

Sources
The New Harvard Dictionary of Music, ed. Don Randel. Cambridge, Massachusetts, Harvard University Press, 1986. ISBN 0-674-61525-5
Articles "Intermezzo," "Intermedio" in The New Grove Dictionary of Music and Musicians, ed. Stanley Sadie. 20 vol. London, Macmillan Publishers Ltd., 1980. ISBN 1-56159-174-2