folk song vs world music

folk song

noun
  • folk music 

  • A song, in any folk style, originating among ordinary people, usually non-professional musicians, and handed down by oral tradition. 

  • A song in the style of folk music, a modern genre of popular music. 

world music

noun
  • Traditional music, as opposed to popular music or classical music. 

  • Music that combines elements from traditional styles from the non-Western world along with modern, popular elements, in order to create something marketed towards Western audiences. 

How often have the words folk song and world music occurred in a corpus of books? (source: Google Ngram Viewer )