1) A character, previously shaped by natural selection for a particular function (an adaptation), is coopted for a new use—cooptation, or
2) A character whose origin cannot be ascribed to the direct action of natural selection (a nonaptation), is coopted for a current use—cooptation.
Such shifts in the function of a trait are referred to as exaptation, cooption, or preadaptation. Feathers provide the classic example of exaptation – having initially evolved for temperature regulation, feathers later evolved for their function in flight. Human hands result from a long sequence of exaptations – fins in fish; forelimbs in transitional forms such as Tiktaalik, Acanthostega, or Ichthyostega; forelimbs in tetrapods; structures for grasping of tree branches in arboreal primates; tool-grasping structures in hominids and humans. The proteins that form the bacterial flagellum are derived from proteins that served other functions in smaller assemblages.
Interest in exaptation results from its relationship to both the process and product of evolution: to the process that creates complex traits and to the product that may be imperfectly designed.
Gould, Stephen Jay, and Elizabeth S. Vrba (1982), "Exaptation - a missing term in the science of form," Paleobiology 8 (1): 4-15.