opus 
/ˈoʊpəs/ noun
plural op·era /ˈoʊpəs/ noun /ˈoʊpərə/ also opus·es /ˈoʊpəsəz/ [count] 1 : a piece of music written by a major composer — usually singular — usually followed by a number (called an opus number) that indicates when a piece of music was written in the list of works written by the same composer ▪ The concert began with Beethoven's Opus 27. 2 formal : an important work done by a writer, painter, etc. — usually singular ▪ the author's latest opus — see also magnum opus
