The baroque era (and for this we are taking the period from around 1600 to the death of Handel in 1759) saw both the birth of opera as a musical form and its growth into perhaps the most enduring musical genre.

Of the thousands of operas which were known to have been written in those 150 years, only a small percentage survive, notably those of such composers as Scarlatti, Handel, Pergolesi, Hasse and Gluck.  Written initially to celebrate specific events (usually royal or vice-regal) this rôle was taken over by the serenata and the developing larger cantata forms and the operas became entities in themselves, drawing large audiences at the many theatres which appeared, creating an almost 'popular culture' with the Europe-wide distribution of new works, and importantly for the later development of singing, creating a need for virtuoso performers which was fulfilled by singers like Farinelli, La Romanina, Bordoni, Cuzzoni, Senesino, Cafarelli, Carestini and those who followed.

In literature some of the greatest writers of the day were creating libretti for the composers to set to music, and one libretto would often be set by up to a dozen composers.  One of the greatest and most influential (even until Mozart's time) was Abbate Pietro Metastasio. His complex and often riveting stories (eg. Didone abandonata, L'Olimpiade & Attilio regolo) were set to music by most of the great baroque opera composers.

The refining of recitativo accompagnato and secco, the development of the aria as a form, subject matter historical or mythological, pathetic or comic and above all the music, the singing and the spectacle made baroque opera a pinnacle of musical acheivement.

OPERA
"...a drama set to music and made up of vocal pieces with orchestral accompaniment and with orchestral overtures and interludes..."*

BAROQUE
The term Baroque probably ultimately derived from the Italian word barocco, which was a term used by philosophers during the Middle Ages to describe an obstacle in schematic logic. Subsequently the word came to denote any contorted idea or involuted process of thought. Another possible source is the Portuguese word barroco (Spanish barrueco), used to describe an irregular or imperfectly shaped pearl, and this usage still survives in the jeweller's term baroque pearl. In art criticism the word Baroque came to be used to describe anything irregular, bizarre, or otherwise departing from established rules and proportions.*

ENCYCLOPÆDIA BRITANNICA

 Detail from Nicola Porpora's Componimento drammatico Gli orti Esperidi