There are four or five accepted and cognate meanings of spandrel in architectural and art history, all relating to the space between a curved figure and a rectangular boundary--such as the space between the curve of an arch and a rectilinear bounding moulding, or the wallspace bounded by adjacent arches in an arcade and the stringcourse or moulding above them, or the space between the central medallion of a carpet and its rectangular corners.
Another usage of the word spandrel was popularized by Stephen Jay Gould and Richard Lewontin in the paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme". In the context of evolution, a spandrel is a metaphor for characteristics that are or were orginally side effects and not true adaptions to the environment.
This metaphorical meaning works no matter which kind of architectural spandrel is referred to: the spandrel is the un-designed gap between other features, which is then often exploited for a use of its own.
Reference: Gould, Stephen Jay and Richard C. Lewontin. "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme." Proc. Roy. Soc. London B 205 (1979) 581-598