solo song with instrumental accompaniment, an important element of opera but also found extensively in cantatas and oratorios. The term originated in Italy in the 16th century and first gained currency after 1602, when Giulio Caccini (Caccini, Giulio) published Le nuove musiche (The New Music), a collection of solo songs with continuo (usually cello and harpsichord) accompaniment. Caccini called his strophic, or stanza-form, songs arie (singular aria). Most serious strophic songs published in Italy after 1602 were called arias, and in 1607 the form made its way into opera, in Orfeo by Claudio Monteverdi (1567–1643).

Instead of using the same music for every stanza, some composers placed variations of a melody over a repeated, steadily moving bass line. Arias of a popular or frivolous cast were often called canzonetta, or arietta. After about 1620, arias nearly always were composed in triple time (e.g., ³/₄) and also were longer and in new musical forms, often suggested by the texts. By the mid-17th century a preference for bi-partite (i.e., AB) forms was superseded by a reliance on the da capo aria, in which the initial melody and text were repeated after an intervening melody and text had been sung (i.e., ABA). Often the inner B section was set in duple time (e.g., ²/₄), the outer A sections in triple time (e.g., ³/₄).

During the later 17th and early 18th centuries, the da capo aria was an extremely popular musical form, particularly as a part of Italian operas and cantatas. Aria texts written to the ABA form became shorter in comparison to strophic songs, with only a few lines to each section, although expansive musical forms were created through much-repeated text. The central B section was usually terse and often in a related key, with contrasting mood and tempo. While the story of an opera was advanced through recitative (dialogue sung in quick, speechlike rhythms), the arias, by contrast, were dramatically static, allowing individual characters to reflect upon the immediately preceding action, after which they perhaps left the stage.

Arias might assume different moods and were classified as aria cantabile (lyric aria), aria di bravura (virtuoso aria), aria parlante (speechlike aria), and so on. These were supposed to be carefully distributed throughout an opera, although such composers as George Frideric Handel and Alessandro Scarlatti did not observe this convention rigidly. The most highly acclaimed singers of the age decorated the reprise of the A section with brilliant improvised embellishments, culminating in an unaccompanied cadenza. The da capo aria was also a staple constituent of cantatas and to a lesser extent of oratorios.

By the late 18th century, a reaction had set in against the da capo form, and it went into sharp decline. Such influential figures as the philosopher Jean-Jacques Rousseau (Rousseau, Jean-Jacques) and the composer Christoph Willibald Gluck (Gluck, Christoph Willibald) protested the da capo aria, objecting to its excessive coloratura (or florid singing), to the dramatic impropriety of returning to the mood of section A after the contrasting mood of section B, and to the absurdity often resulting from the repeated section of text.

The aria continued to be prominent in opera after about 1770, but in many different, less stereotyped musical forms, ranging from simple strophic songs to long, elaborate scenes. The operas of Gluck were the first important ones to utilize such a variety of arias. The aria also enjoyed a vogue as a concert piece. Operatic arias (e.g., Leporello's “Catalogue Aria” in W.A. Mozart's Don Giovanni) were often written in two parts, one dramatic and one lyrical.

In Italian opera up to Aida (1871), the aria was cultivated over a longer period than in German opera. Richard Wagner (Wagner, Richard) in his operatic reforms utilized a continuous musical texture in place of separate numbers, using arias as songs only in special instances (e.g., the “Prize Song” in Die Meistersinger). In the 20th century, arias occurred largely in operas by composers uninfluenced by or hostile to Wagner (e.g., Igor Stravinsky's Rake's Progress and the operas of Benjamin Britten). The word aria is occasionally used for instrumental pieces of a songlike nature, as the two middle movements of Stravinsky's Violin Concerto.