This is what we found on Wikipedia regarding the Windborne:

Music performed a cappella ( AH kə-PEL-ə, UK also AK ə-PEL-ə; Italian: [a kkapˈpɛlla]; lit. 'in [the style of] the chapel'), less commonly spelled a capella in English, is music performed by a singer or a singing group without instrumental accompaniment. The term a cappella was originally intended to differentiate between Renaissance polyphonic and Baroque concertato musical styles. 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, rarely, as a synonym for alla breve.