A functional adaptation is a structure or behavior that has arisen sometime in the evolutionary history of a species to aid in that species', or its predecessors', survival. A waterproof eggshell, for example, is a functional adaptation that arose among reptiles in response to the need to lay eggs in dry, dessicating environments where permeable eggshells would have dried out.
Functional adaptations are at the heart of evolution. Structures that increase the likelihood of survival for an organism are strongly favored by natural selection, so they appear in ever-greater numbers from one generation to the next until the trait becomes fixed and every member of the population has them. Sometimes functional adaptations make the transition from one function to another. An example of this is feathers. Feathers arose during the Mesozoic as a form of insulation for some dinosaur species. They were functional as insulation in these species. In time, the shape of some feathers changed to allow flight among the ancestors of modern birds. Feathers thus took on a new function during the course of bird evolution.
Once-functional adaptations can lose their function. These vestigial adaptations are not always useless. The human coccyx, or tailbone, is an example of a vestigial adaptation. It still serves as an attachment point for muscles but is regarded as having greatly diminished functions from ancestral forms.