Spandrel

A spandrel is the space between two arches. It is the space between an arch and a rectangular fence or the space between the right and left exterior curve of an arch and an enclosing right angle.
In structures with more than one floor, the term "spandrel" can also mean the space between the top of the window in one floor and the threshold of the window in the floor above.
Spandrels have different meanings in architecture and art history.
Bridges.
Arches commonly exist in bridge constructions, so spandrels may appear in such structures. Historically, most arch expansions had solid spandrels, meaning that the areas between arches were completely filled in with masonry. This was true until the use of steel and reinforced concrete began in the 19th and 20th centuries.
Domes.
Structures of domes can also contain spandrels. Where domes are needed to rest on a square or a rectangular base, the dome is raised above the level of the supporting pillars with three-dimensional spandrels called pendentives. These triangular segments take the weight of the dome and place it onto the poles.