1) Automotive styling feature 
Opera windows are small porthole sized side windows in the C-pillar of some cars. Typically offered in unison with a vinyl roof, they were a very common design feature of American automobiles during the 1970s. The design was new at the time, `... and would prove to be very popular, indicated by its imitation by almost every domestic manufacturer.....
a narrow, fixed window on each side of the rear passenger compartment of an automobile.