Read this: A cappella [a kapˈpɛlla] (Italian for "in the manner of the chapel") music is specifically group or solo singing without instrumental accompaniment, or a piece intended to be performed in this way. It contrasts with cantata, which is 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.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): At what point did Renaissance polyphony become popular again?
19th century