ballad vs opera

ballad

noun
  • A kind of narrative poem, adapted for recitation or singing; especially, a sentimental or romantic poem in short stanzas. 

  • A slow romantic song. 

verb
  • To compose or sing ballads. 

opera

noun
  • A theatrical work, combining drama, music, song and sometimes dance. 

  • A building designed for the performance of such works; an opera house. 

  • The score for such a work. 

  • Any showy, melodramatic or unrealistic production resembling an opera. 

  • A company dedicated to performing such works. 

How often have the words ballad and opera occurred in a corpus of books? (source: Google Ngram Viewer )