In psychology, habituation refers to learned behavior in ignoring neutral stimuli. Habituation theory holds that when an animal is repeatedly exposed to stimuli that neither hurts nor helps, it stops responding. This lack of response to something that isn't posing a problem means that the animal isn't wasting energy; it's still alert in case danger does occur.
Although it's easy to confuse this type of learning with sensory adaptation, the two aren't the same. Behavior is the key in habituation, as the animal may respond to the stimuli in other ways, but one part of the response is stopped. The animal learns to ignore something that doesn't matter. Sensitization creates an increase in response, while habituation causes a decrease. The decrease may be gradual.
For example, a chipmunk in a park may voice an alarm sound and run up a tree when human footsteps approach. After repeated times when the footsteps eventually faded away and people just kept walking by, the chipmunk may have run up the tree without making any vocal sound. Eventually, if a threat to its safety still hasn't occurred as the footsteps continued, the chipmunk may not run up the tree or make any sound. Its response to the stimuli decreases gradually. The change in behavior is a learned response to avoid wasting unneeded energy.
Habituation means that when something doesn't pose a threat to our safety, we get used to it. We learn to just put up with harmless stimuli rather than waste our energy reacting to it. For instance, if a person moves into a home near a railroad track and the vibration can be felt through the floor every time a train goes by, at first he or she may feel like something bad will happen, such as the vibration will cause people in the home to fall or an object to fall and break. After repeated exposure to the train's vibration when nothing bad ever happens, the person experiences habituation and no longer behaves in a worried manner when a train goes by the home.
Drug habituation is something different. In this case the habit to respond to stimuli that feels beneficial occurs. Eventually the effect of the drug may be neutral, but the effects without the stimuli, specifically withdrawal symptoms, are so negative, that the habit remains. Drug addiction can be both physical and psychological.