The Collaborative International Dictionary of English v.0.48:

Opus \O"pus\, n.; pl. Opera. [L. See Opera.]
   A work; specif. (Mus.), a musical composition.
   [1913 Webster]

   Note: Each composition, or set of pieces, as the composer may
         choose, is called an opus, and they are numbered in the
         order of their issue. (Often abbrev. to op.)
         [1913 Webster]

   Opus incertum. [L.] (Arch.) See under Incertum.
      [1913 Webster] Opuscle

WordNet (r) 3.0 (2006):

opus
    n 1: a musical work that has been created; "the composition is
         written in four movements" [syn: musical composition,
         opus, composition, piece, piece of music]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "opus":
   Nachtmusik, absolute music, adaptation, air varie, aleatory,
   aleatory music, arrangement, article, artifact, autograph,
   best seller, book, bound book, brainchild, chamber music,
   chamber orchestra, child, classic, coinage, coloring book,
   composition, computer printout, concoction, copy, creation,
   creature, crowning achievement, definitive work, descant,
   distillation, document, draft, edited version, effect,
   electronic music, end product, engrossment, essay, essence, etude,
   exercise, extract, fair copy, fiction, final draft,
   finished version, first draft, flimsy, folio, fruit, great work,
   handiwork, hardback, harmonization, holograph, incidental music,
   instrumental music, invention, issue, juvenile, juvenile book,
   letter, limp-cover book, literae scriptae, literary artefact,
   literary production, literature, lucubration, magnum opus,
   manufacture, manuscript, masterpiece, masterwork, matter, mintage,
   new mintage, nocturne, nonbook, nonfiction, notebook, novel,
   oeuvre, offspring, opera, opuscule, opusculum, orchestration,
   original, origination, outcome, outgrowth, paper, paperback,
   parchment, penscript, picture book, piece, piece of writing, play,
   playbook, pocket book, poem, prayer book, printed matter, printout,
   product, production, program music, psalmbook, psalter,
   publication, reading matter, recension, result, ricercar, score,
   screed, scrip, script, scrive, scroll, second draft, serial,
   sketchbook, soft-cover, sonata, sonatina, songbook, standard work,
   storybook, string orchestra, string quartet, study,
   the written word, theme and variations, title, tome, trade book,
   transcript, transcription, trio, typescript, variation, version,
   volume, work, writing



The Free On-line Dictionary of Computing (30 December 2018):

Opus

    A Honeywell operating system promised
   as a sop to customers after canning Multics in 1985.  Opus
   was to provide everything Multics had and more, plus total
   compatibility with the Level 6/DPS6 operating system.

   "Opus" was a code name, the system was officially named VS3
   (short for HVS R3 or Honeywell Virtual System Release Three).
   It was to run on the DPS6-plus hardware known internally as
   the MRX and HRX, and be all things to all people.

   The hardware was a dud (though it did run the native DPS6
   software just fine), and the goal was, shall we say,
   ambitious.  The effort was cancelled by Bull in 1987, in
   favor of another project going on in France.