An arioso is a musical composition usually designed for a solo singer, most often associated with opera singing. It adopts the speech-like patterns and declamatory style of recitative but is slightly more metrical. It is also more like an aria in melodic form.
As opera developed in the 17th century, particularly in Italy around Naples, composers used recitative, or singing that takes the timing and natural flow of speaking, to demonstrate what characters felt about their situations. At first, recitatives kept as close to speech as possible. People thus called this type of singing dry recitative or recitative semplice, or simple recitative. Later, composers used the orchestra to dramatize moments in the recitatives. Finally, composers began to make recitatives freer and more melodic, thereby developing the arioso.
Roman musicians clearly had defined solo singing into two categories — speech-like recitative and melodic aria — by the first half of the 1600s. The problem was that no real middle ground existed between the two forms. Composer Domenico Mazzocchi tried to balance recitative and aria with mezz'arie, or "half arias," which were brief melodic interludes within the larger recitative. Even so, this approach was more like interjection of free melody. When arioso finally developed, it at last created a true middle ground between recitative and aria singing.
Compared to aria, arioso typically is shorter, and also tends to be freer in form. This makes it an excellent transition between recitative and aria passages. Composers may use arioso by itself, however, using it to replace a recitative or aria. This depends on the preferences of the composer and the context of the scene the composer is trying to depict.
Arioso is found in different types of musical works. For instance, it is traditionally a component of opera, but composers also use it with great success in oratorios and cantatas. This means it is better to think of arioso as a style of singing, rather than to associate it with any particular genre.
The fact arioso is a style and not a genre is further exemplified in the instrumental adoption of this type of musical performance. Once arioso was firmly developed, instrumentalists routinely arranged ariosos so they could enjoy the vocal works. Understanding that instrumentalists could adopt the characteristics of ariosos as they played, composers began to compose ariosos directly for instruments in the same manner they had composed them for singers, even including the word "arioso" in the titles of the instrumental compositions. The composer's expectation that the musician would perform the work with a great deal of expression allowed instrumentalists to have some rhythmic freedom without entirely eliminating the boundaries within designated musical meters.