OPUS

Noun

opus (plural opuses or opera)

1. (music) A work of music or set of works with a specified rank in an ordering of a composer’s complete published works.Beethoven’s opus eighteen quartets are considered by many to be the beginning of the Romantic era.
2. A work, especially of art.The painter’s last opus was a dedication to all things living, in a surprising contrast to all of his prior work.

Usage notes

The most common plural of opus in English is opuses. Some people use the Latin plural, opera. Opi is fairly common in the field of classical music, though mostly in informal contexts. The use of any of these three pluralizations may result in the speaker being corrected, though opi, above all, should be avoided in formal contexts. Outside of music, the word opus sees particularly frequent use in the expression magnum opus.

Source: https://en.wiktionary.org/wiki/opus