What is "opera window"

n. A small porthole-sized side window in the C-pillar of certain cars, popular in the 1970s.

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 "... would prove to be very popular, indicated by its imitation by almost every domestic manufacturer. The opera window was a fixed rear side window surrounded by a vinyl roof."