A cappella music is specifically solo or group singing

Singing

Singing is the act of producing musical sounds with the voice, and augments regular speech by the use of both tonality and rhythm. One who sings is called a singer or vocalist. Singers perform music known as songs that can be sung either with or without accompaniment by musical instruments...

 without instrumental

Musical instrument

A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

 sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

, or a piece intended to be performed in this way. It is the opposite of cantata

Cantata

A cantata is a vocal composition with an instrumental accompaniment, typically in several movements, often involving a choir....

, which is accompanied singing. A cappella was originally intended to differentiate between Renaissance

Renaissance music

Renaissance music is European music written during the Renaissance. Defining the beginning of the musical era is difficult, given that its defining characteristics were adopted only gradually; musicologists have placed its beginnings from as early as 1300 to as late as the 1470s.Literally meaning...

 polyphony

Polyphony

In music, polyphony is a texture consisting of two or more independent melodic voices, as opposed to music with just one voice or music with one dominant melodic voice accompanied by chords ....

 and Baroque concertato

Concertato

Concertato is a term in early Baroque music referring to either a genre or a style of music in which groups of instruments or voices share a melody, usually in alternation, and almost always over a basso continuo...

 style. In the 19th century a renewed interest in Renaissance polyphony coupled with an ignorance of the fact that vocal parts were often doubled by instrumentalists led to the term coming to mean unaccompanied vocal music.