These terms are used by psychologists to describe how an organism's responses to stimuli change over time, based on experience to differing stimulus exposure intensities and frequencies. It figures in theories of perception as well as learning. As I understand it, adaptation refers to a decreased response of sensory receptors to a given stimulus intensity, and this is mostly a peripheral process. Habituation implies a higher more central change in the stimulus-response circuitry that is more like what we think of as "learning." Desensitization is a term that is used in psychology to describe a decreased response to noxious or otherwise unpleasant stimuli, without necessarily a decreased sensitivity of the sensory receptors themselves.
