\ ˈō-pəs
\
plural opera\ ˈō-pə-rə
, ˈä- \
also opuses\ ˈō-pə-səz
\
: work
especially
: a musical composition or set of compositions usually numbered in the order of its issue
History and Etymology for opus
borrowed from Latin oper-, opus (plural opera) "work, effort, product of labor, work of art," going back to Indo-European *h3ep-os-, *h3ep-es- "work" (whence also Sanskrit apas- "work, action"), derivative of *h3ep- "ability, force," whence Latin op-, *ops "power, ability, wealth, resource(s)," and, with varying suffixation, Sanskrit apnas- "possession, property, work," Hittite happina- "rich," happir-, happar- "business, trade," Germanic *afla- (whence Old English afol "power, might," Old Norse afl "strength"); and, with lengthened ablaut grade, Sanskrit āpas- "work, religious act," Avestan huuāpah- "performing good deeds," Germanic *ōbjan- "to perform" (whence Old Saxon oƀian "to celebrate," Old High German uoben, uoppen "to practice, exercise")
Note:
Possibly also related are Old Norse efna "to perform, fulfill," Old English efnan "to accomplish, achieve" (from Germanic *abnjan-?), but this is uncertain.