An adaptation is a feature that is common in a population because it provides some improved function. Adaptations are well fitted to their function and are produced by natural selection.
Adaptations can take many forms: a behavior that allows better evasion of predators, a protein that functions better at body temperature, or an anatomical feature that allows the organism to access a valuable new resource all of these might be adaptations. Many of the things that impress us most in nature are thought to be adaptations.
So what's not an adaptation? The answer: a lot of things. One example is vestigial structures. A vestigial structure is a feature that was an adaptation for the organism's ancestor, but that evolved to be non-functional because the organism's environment changed.
Fish species that live in completely dark caves have vestigial, non-functional eyes. When their sighted ancestors ended up living in caves, there was no longer any natural selection that maintained the function of the fishes' eyes. So, fish with better sight no longer out-competed fish with worse sight. Today, these fish still have eyes but they are not functional and are not an adaptation; they are just the by-products of the fishes' evolutionary history.
In fact, biologists have a lot to say about what is and is not an adaptation.
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Katydid image © Greg Neise, GE Neise Digital Communication; Creosote image courtesy of US Geological Survey; Cave fish image courtesy of Wetland Care Australia
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California