Term for a song either independent or part of a larger work. The Italian word may be rendered as 'style' or 'manner', and in the 16th century 'aria' was used for simple settings of light poetry (e.g. 'aria napoletana'). Arias as melodies or schemes for songs were printed during much of the 16th century and well into the 17th in instrumental as well as vocal publications.

The aria had a central place in early opera, cantata and oratorio. Most Venetian opera arias before 1660 are in triple time or a mixture of triple and duple; many early arias have four or more verses, though after 1650 two became the standard in opera. Most arias have continuo accompaniment, with instrumental ritornellos between verses; a few from the 1640s onwards have instrumental sections between vocal phrases, but these remain a minority until well into the 18th century.

Most later 17th-century arias are in the form A B B' (the last line or group of lines rendered twice to similar music, with a tonic cadence only the second time), or A B A (sometimes A B A'), where the first line or couplet is repeated at the end. This became the standard da capo aria, which was dominant by 1680. In the early 18th century the accompaniment could vary in texture and instrumentation; the aria with continuo only became increasingly rare after the 1720s.

By then, longer arias were favoured. With the composers regarded as originators of the modem 18th-century style (Vinci, Hasse, Pergolesi etc), the proportions of the da capo structure changed. The middle section became shorter and often contrasted in tempo and meter; the corresponding enlargement of the first section later led to the practice of replacing the da capo with the dal segno, indicating a return not to the beginning but to a later point. In the 1760s and 1770s this gave way to a scheme close in outline to the contemporary symphony or sonata first movement, with a first section ending in the dominant, a middle section as development or contrast and a restatement of the first section as tonic recapitulation. Other important types of this time were the rondeau, ABACA, and the so-called rondo, which began with a slow section and ended with an allegro (AB or ABAB). By 1780 the latter (prototype of the early 19th-century cantabile-cabaletta) had largely replaced the one-tempo French variety. Arias in comic operas were more varied in form.

19th-century operas show a continuing reduction in the number of arias and an increase in their length. The sonata-form aria gave way to multi-tempo forms and there was a move from the older bel canto style towards a more dramatic one from the 1830s. Verdi's development exemplified the move towards free and fluid constructions that cannot readily be extracted from their context. In Puccini too the aria tends to become part of the dramatic texture; and in Wagner's mature operas the extended sections for single voice cannot usually be extracted without mutilation.

Italian opera strongly influenced most other contemporary operatic genres, including French grand opera and Slavonic opera, in which the aria was accepted as a natural form of expression. Wagner's influence, however, was such that by the 20th century older traditions had been virtually discarded. Stravinsky, in "The Rake's Progress" (1951), revived the form but not the substance of the 18th-century aria; more recent tendencies have been towards highly integrated forms of music theatre from which the aria has usually been excluded.

In the early 17th century, and even as late as Bach's Goldberg Variations, an 'aria' sometimes served for a set of instrumental variations, and pieces called 'aria' (often in bourrée rhythm) were common in ensemble dance music in the Baroque.