A trait that evolve
d under pressure to fulfill a different function than the one it currently serves.
Exaptation serves as a note of caution against uniformitarianism, the idea that the processes observable in the present can be used ot explain the past. The greatest example of this is the amphibious egg. It was a evolutionary conundrum how the egg went from the soft, permeable membrane of the ocean eggs to a solid impermiable shell on land apparently in one gigantic evolutionary leap. After all, an egg cannot be somewhat permeable and suvive on land. It would simply dry out.
It was later discovered that the egg evolved through a semi-permeable state in its evolution "in an effort" to survive in fresh water. Fresh water would also dry out the egg because of the differentiation in salt, but a semi-permeable membrane was sufficient to control the flow. River mouths and deltas create a continuum of "saltiness" that would allow the change in permiability to happen slowly.