An “exaptation” is just one example of a characteristic that evolved, but that isn’t considered an adaptation. Evolutionary biologists Stephen Gould and Elizabeth Vrba proposed vocabulary to let biologists talk about features that are and are not adaptations:

• Adaptation — a feature produced by natural selection for its current function (such as echolocation in bats, right).
• Exaptation — a feature that performs a function but that was not produced by natural selection for its current use. Perhaps the feature was produced by natural selection for a function other than the one it currently performs and was then co-opted for its current function. For example, feathers might have originally arisen in the context of selection for insulation, and only later were they co-opted for flight. In this case, the general form of feathers is an adaptation for insulation and an exaptation for flight.