Architecturally, a spandrel (less often spandril or splaundrel) is the space between two arches or between an arch and a rectangular enclosure. As a metaphor in evolutionary biology, a spandrel is a phenotypic characteristic that evolved as a side effect of a true adaptation.
In biology, 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 originally side effects and not true adaptations to the environment. They are analogous to misbugs in hacker jargon.
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.