Definitions of Adaptation

(Modified from:  Futuyma, 1998)

I) As used by physiologists: 

A)   Adaptation describes an individual organisms' phenotypic adjustments  to the environment; the gradual and reversible adjustment of physiology or morphology as a result of environmental stresses.

II) As used by evolutionary biologists:

A) Adaptation can be used to refer to a process of becoming adapted (as in "adaptation cannot occur without genetic variation"). 

B) Adaptation can be used to refer to the features of an organism than enhance reproductive success (including survival) relative to another possible feature.  That is, an adaptation is any feature, trait or attribute that increases fitness.