In music, a solo (from the Italian: solo, meaning alone) is a piece or a section of a piece played or sung by a single performer. Performing a solo is "to solo", and the performer is known as a soloist.
The plural is soli or the anglicised form solos. In some context these are interchangeable, but soli tends to be restricted to classical music, and tends to refer to either the solo performers or the solo passages in a single piece: it would not often be used to refer to several pieces that happen to be for single performers. Furthermore, the word soli can be used to refer to a small number of simultaneous parts assigned to single players in an orchestral composition. In the Baroque concerto grosso, the term for such a group of soloists was concertino.
In the Baroque and Classical periods, the word solo was virtually equivalent to sonata, and could refer either to a piece for one melody instrument with (continuo) accompaniment, or to a sonata for an unaccompanied melody instrument, such as Johann Sebastian Bach’s sonatas for violin alone.