"ADAPTATION" can be a confusing term because of the many ways it is used. For example, "adaptation" can refer to both processes and products:
(2) an evolutionary change such as those caused by selection pressures on some attribute of a trait that helps the organism cope with a changing environment over the generations, or
(3) a biological TRAIT that exists because it confers or is linked to a trait that now (or in the past) has conferred a biological advantage enhancing an organism's FITNESS.
SO, when we speak of an organism's adaptations we are referring to traits subject to natural selection which persist because they contribute to fitness (or at least, contribute more than they cost). Adaptations are the means by which organisms cope with environmental changes and stresses.
An Adaptation can be manifest at any level of organization from subcellular through the ecosystem in which any level of organism -- environment conformity can be discerned. Within a single organism, "adaptation" can encompass morphology, physiology, development (through organizational effects or through differential timing of developmental events), and behavior.
ADAPTATION is a complex term because of the many ways it is used. It can refer to a TRAIT that confers some FITNESS on an animal, BUT it also represents the PROCESS by which that trait has come about.
"adaptations are traits (or characters) that have been subjected to natural selection" This means that the trait has "evolved" (been modified during its evolutionary history) in ways that have contributed to the FITNESS of the organism manifesting it .
An adaptation is an anatomical, physiological, or behavioral trait that contributes to an individual's ability to survive and reproduce ("fitness") in competition with conspecifics in the environment in which it evolved (Williams, G. 1966. Adaptation and Natural Selection Princeton).
"The processes by which organisms or groups of organisms maintain homeostasis in and among themselves in the face of both short-term environmental fluctuations and long-term changes in the composition and structure of their environments." (Rappaport, 1971).