This type of music are specifically group or solo singing without any Insturmental accompainiment, or a piece intended to be in such this kind of way.
It contrasts with cantata, which is usually accompanied singing. The term "a cappella" was originally intended to differentiate between Renaissance polyphony and Baroque concertato 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. The term is also used, albeit rarely, as a synonym for alla breve.