Folk music is a music genre that includes traditional folk music and the contemporary genre that evolved from the former during the 20th century folk revival. Some types of folk music may be called world music. Traditional folk music has been defined in several ways; as music transmitted orally, music with unknown composers, music that is played on traditional instruments, music about culture or national identity, music that changes between generations (folk process), music associated with a people's folklore, or music performed by custom over a long period of time. It has been contrasted with commercial and classical styles. The term originated in the 19th century, but folk music extends beyond that.