Opus is a term used in numbering a composer’s
works. For example, Mozart’s Opus 50 is theoretically
his fiftieth published composition.
Opus numbers are not always accurate in that sense, however,
since the composers and publishers who’ve done the numbering
have often purposely left certain compositions out.
While the term opus is generally reserved for music, the
term magnum opus may refer to a large and important work
in any discipline, as well as to the most important work of a
composer, artist, or writer.
For example, some critics consider the opera Don Giovanni
to be Mozart’s magnum opus.