What are opus?

Some articles on opus:

String Sextet No. 1 (Brahms) - Sextet Chronology
... basse from the 1780s (still later than the 1776 or so of Boccherini's opus 23) by Ignaz Pleyel, Ignacy Feliks Dobrzyński's opus 39 in E-flat (from 1845 or 1849?), Louis Spohr's in C opus ...
List Of Compositions By Henryk Górecki - Works - Published
... Four Preludes, Opus 1, piano (1955) Toccata for two pianos, Opus 2 Three Songs, Opus 3 (1956) Variations for violin and piano, Opus 4 Quartettino, Opus 5, two flutes, oboe, violin (1956) Piano Sonata No. 1, Opus 6, piano (1956, revised 1984, 1990) Songs of Joy and Rhythm, Opus 7, piano and chamber orchestra (1956, revised 1959) Sonatina in One Movement, Opus 8, violin and piano Lullaby for piano, Opus 9 (1. 1 '1959', Opus 14, string orchestra and percussion (1959) Three Diagrams for solo flute, Opus 15 Monologhi, Opus 16, soprano and three groups of instruements (196 ...
CELT
... by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF) ... stand-alone CELT has been merged into Opus ... is going on only for its hybridised form as a layer of Opus, integrated with SILK ...
Opus, Greece
... Opus (also Opous, Ancient Greek Ὀποῦς), in Ancient Greece, the chief city of Opuntian or Eastern Locris ... In the Iliad, Homer mentions Opus as one of the Locrian cities whose troops were led by Ajax the Lesser, son of Oileus the king of Locris (Homer, Iliad, 2.525-530) ... Pindar's ninth Olympian ode, concerns Opus ...
Catalogues Of Beethoven Compositions - Opus Number
... works was for publishers to assign some sort of consecutive Opus number to the works as they were published ... cannot reliably identify unpublished works, nor do they have a reliable indication from the opus number of the actual composition date(s) ... the Octet, written from 1792–1793 is Opus 103, while Opus 102 and Opus 104 were written in 1815 and 1817 respectively ...