a cappella vs opera

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

  • Related to a form of purely vocal music mostly associated with American college performance groups. 

adv
  • In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

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 a cappella and opera occurred in a corpus of books? (source: Google Ngram Viewer )