opus (n.) Look up opus at Dictionary.com
"a work, composition," especially a musical one, 1809, from Latin opus "a work, labor, exertion" (source of Italian opera, French oeuvre, Spanish obra), from PIE root *op- (1) "to work, produce in abundance" (Germanic *ob-) "to work, produce in abundance," originally of agriculture later extended to religious acts (cognates: Sanskrit apas- "work, religious act;" Avestan hvapah- "good deed;" Old High German uoben "to start work, to practice, to honor;" German üben "to exercise, practice;" Dutch oefenen, Old Norse æfa, Danish øve "to exercise, practice;" Old English æfnan "to perform, work, do," afol "power"). The plural, seldom used as such, is opera.