The process by which a trait evolves for one function and is later co-opted for another is called exaptation. The term describes a feature's current evolutionary use, regardless of whether that use was the reason the feature originally evolved.
A classic example involves feathers. Early feathers may have evolved for insulation, display, or other functions before they were adapted for powered flight in birds. Their later role in flight was therefore an exaptation rather than necessarily the original purpose of feathers.
The concept was developed in a 1982 paper by Stephen Jay Gould and Elisabeth Vrba. They wanted to distinguish traits shaped by natural selection for their present role from traits that originated for another reason and were later recruited for a new function. Exaptations can also involve structures that arose as side effects of development rather than as direct adaptations.
“Preadaptation” is an older term sometimes used for similar cases, but it can misleadingly suggest that evolution anticipated a future need. A spandrel is a by-product or incidental feature, while adaptation is the broader term for a trait favored for its function.