A spandrel is a useful characteristic that did not orginally come about for their current use. Rather, a spandrel is a by-product of the evolution of some other characteristic.
In the basillica St. Mark's in Venice, there are beautifully decorated triangular spaces in the spot where the archways that support the roof meet. This is a great use of the space, but the intention of the space was not designed for that purpose. The lesson here is, when a trait that has a good function, don't just assume the trait was desinged for that function. The trait may be a by-product of the evolution of another characteristic. A spandrel, as the space between two archways is called, is also a term used in evolution. Stephen J. Gould is an evoluntionist who has written about evolutionary spandrels. In Gould and Lewontin's paper "The Spandrels of San Marcos and the Panglossian paradigm: a critique of the adaptationist programme", spandrels were used as an example to explain why adaptationists were wrong (Gould & Lewontin, 1979). (Adaptationists believe every trait came about as the result of natural selection).
Gould, S. J., & Lewontin, R. C. (1979). The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society of London. Series B. Biological Sciences, 205(1161), 581-598.
Some examples of spandrels include:
The red color of red blood cells (they contain hemoglobin for transporting oxygen and carbon dioxide)
The pseudo-penis of hyenas (now considered an exaptation)
The by-products of the large human brain: religion, reading, writing, fine arts, etc. (Gould, 1991)