Opera window

An opera window is a small fixed window usually behind the rear side window of an automobile. They are typically mounted in the C-pillar of some cars. The design feature was popular during the 1970s and early 1980s that was adopted by domestic U.S. manufacturers most often with a vinyl roof.
The origin was from around 1915 "opera" vehicles with occasional collapsible seating for extra passengers. The opera window was also a feature on "formal roof" and limousine models with higher than normal roof to accommodate passengers with top hats.
History.
The design element of a distinct, fixed, centered opera window was borrowed from such windows in horse-drawn carriages and used during the classical era of automobile styling. For example, “...the Elcar in 1924 was good looking...and even a fabric top in the style of a brougham with oval opera windows framed by landau bars... Opera windows saw their demise in the 1930s. 
Perhaps the most notable return was the "porthole" in the 1956–1957 Ford Thunderbird. It was provided as an option to improve rear-quarter visibility with the removable hardtop in place. “...The hottest thing going was the "porthole" window in the rear side pillar - called "opera windows" that came in during the horse and buggy [era]...
Opera windows began reappearing in the early 1970s in such vehicles as the 1972 Continental Mark IV. Almost all personal luxury cars would adopt-opera windows, usually framed by a vinyl roof. Most often, opera window variants were applied on two-door hardtop or coupé models, spanning all types of vehicles from economy compacts to flagship personal luxury cars, in which latter exploding realm they became “recognition elements” seeking to add a vintage element to their styling. General Motors introduced all-new line of mid-sized "Colonade" models for the 1973 model year. Standard on all the coupes was a fixed triangular rear quarter window while higher trim versions used a rectangular vertical opera window. 
In some cars, an additional feature was the so-called opera light that was mounted on the outside of the B-pillar or C-pillar and illuminated when the exterior lights were switched on.
Function.
The windows were intended to offset the significant blind spots created by wide C-pillars that were characteristic of many American cars produced at this time. In an age of decreasing dimensions and increasingly common use of non-opening rear side windows on 2-door models, a variety of shapes of rear windows may have helped passengers there to feel somewhat less claustrophobic.
These windows were usually non-functional; however, in the case of the AMC Matador coupe NASCAR racers, the small windows that came with the Barcelona II trim package actually helped to clean up the aerodynamics when such windows were open to the wind under racing conditions.