We suggest that such characters, evolved for other usages (or for no function at all), and later ›coopted‹ for their current role, be called exaptations. […] They are fit for their current role, hence aptus, but they were not designed for it, and are therefore not ad aptus, or pushed toward fitness. They owe their fitness to features present for other reasons, and are therefore fit (aptus) by reason of (ex) their form, or ex aptus. Mammalian sutures are an exaptation for parturition. Adaptations have functions, exaptations have effects. The general, static phenomenon of being fit should be called aptation, not adaptation. (The set of aptations existing at any one time consists of two partially overlapping subsets: the subset of adaptations and the subset of exaptations […])
Gould, S.J. & Vrba, E.S. (1982). Exaptation – a missing term in the science of form. Paleobiol. 8, 4-15: 6.