magnum opus

 
Pronunciation: /ˌmagnəm ˈəʊpəs, ˈɒpəs/

noun (plural magnum opuses or magna opera /ˌmagnə ˈəʊpərə, ˈɒpərə/)

  • a work of art, music, or literature that is regarded as the most important or best work that an artist, composer, or writer has produced.

Origin:

late 18th century: from Latin, 'great work'