See caption
Spandrels decorated with figures of winged victories, Arc de Triomphe du Carrousel, Paris
Wikimedia Commons

A spandrel is the roughly triangular space above and on either side of an arch.[1] The wall above the arch, forming the spandrel, is known as the spandrel wall.[2]

In a building with more than one floor, the term spandrel is also used to describe the space between the top of the window in one storey and the sill of the window in the storey above.[3]