"a drama sung" [Klein], "a form of extended dramatic composition in which music is essential and predominant," 1640s, from Italian opera, literally "a work, labor, composition," from Latin opera "work, effort" (Latin plural regarded as feminine singular), secondary (abstract) noun from operari "to work," from opus (genitive operis) "a work" (from PIE root *op- "to work, produce in abundance"). Explained in "Elson's Music Dictionary" as, "a form of musical composition evolved shortly before 1600, by some enthusiastic Florentine amateurs who sought to bring back the Greek plays to the modern stage."
No good opera plot can be sensible. ... People do not sing when they are feeling sensible. [W.H. Auden, 1961]
As a branch of dramatic art, it is attested from 1759. First record of opera glass "small binoculars to aid vision at the theater" is from 1738. Opera-house, "theater devoted chiefly to opera performances," is from 1720.