1. A process in which a feature acquires a function that was not acquired through natural selection.
2. A feature having a function for which it was not originally adapted.
3. A morphological or physiological feature that predisposes an organism to adapt to a different environment or lifestyle.
Etymology: coined in the early 1980s by Stephen Jay Gould and Elizabeth Vrba, from ex + adaptation.
I ran across the word in this article about goldfish. And here is the Wikipedia article. And, a Scentific American article.