A “spandrel” is an “evolutionary by-product.” It is an unrelated trait that travels with another trait, sometimes for totally unknown reasons. One possible example of this is the color of cats: certain fur patterns or colors are linked to higher levels of aggressiveness, but it is unclear why that would be true.
The word originally meant the space between two arches. It was adapted to cover the biological phenomenon by Stephen Jay Gould and Richard Lewontin, in a 1979 essay. The essay (which I haven’t had a chance to read) is here.