An Intermezzo is a musical term describing a composition that fits between other musical or dramatic entities. A way to think of an intermezzo is like an act in a play, the act is a small part that fits between other acts, but all of them together make up the play. In musical history this term has several different usages, but most fit into two different categories: opera intermezzi and instrumental intermezzi. A lot of intermizzi are composed in composite ternary form.
During the 18th century the Opera Intermezzo was usually a comic operatic interlude inserted in between acts or scenes of an opera seria. Although these intermezzi could be substantial and grow out to become works of their own, they were typically smaller works than the opera seria that surrounded them, providing comic relief to the audience after seeing the dramatic tone of the bigger opera surrounding the intermezzi. Often these intermezzi will use one or more of the stock characters in a commedia dell'arte. Also it was common to see these intermezzi characterized as slapstick comedy, wearing ridiculousd costumes, "plain dumb" dialog (as well as dialect), and ribaldry. The most famous of all intermezzi from the period is Pergolesi's La serva padrona, which was an opera buffa that after the death of Pergolesi kicked off the Querelle des Bouffons.
During the start of the 19th century the intermezzo acquired a new meaning. It was usually a smaller work that stood in between two larger works, but also could be substantial to be performed on it's own. These intermezzi show a wide variation in the style and function: in Mendelssohn's incidental music to A Midsummer Night's Dream the intermezzo serves as musical connecting material for action in Shakespeare's play; in chamber music by Mendelssohn and Brahms, the intermezzi are names for interior movements which would otherwise be called scherzi; and the piano intermezzi by Brahms, some of his last compositions, are sets of independent character pieces not intended to connect anything else together. Stylistically the intermezzi of the 19th century were typically melodic, slow, and more relaxed than the piece that surrounded them. The most famous of this type of intermezzo is probably the intermezzo from Cavalleria Rusticana, by Pietro Mascagni. Puccini also wrote intermezzi for Manon Lescaut and Madama Butterfly, and examples exist by Wolf-Ferrari, Delius and others.
During the 20th century and beyond this term becomes more and more uncommon. 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.