Habituation is a process whereby an individual or animal is subjected to the same stimuli repeatedly until it ceases to respond as it initially did. An example of this would be the hardening of an individual to physical pain after repeatedly being abused in the same manner. Eventually, the victim or test subject will grow immune to the procedure and begin to ignore it. He may still feel pain, but he will be accustomed to it and react less demonstrably than was initially the case.
Animals, especially rats, are frequently used in habituation experiments to test their reactions to outside stimuli. Acclimating an animal to a new environment requires consistency in terms of that animal experiences on a daily basis. As the animal becomes accustomed to a certain of activity and surroundings, it will become more comfortable and less agitated.
As sort of a side note, governments have been known to use the process of habituation to lure their enemies in other countries into a false sense of security. In 1941, Joseph Stalin grew so accustomed to the large German armored formations positioned on the border separating Germany from the Soviet Union that he was genuinely surprised when those formations crossed the border into Russia. Similarly, the Egyptian Army repeatedly threatened to invade Israel in 1973, prompting the Israelis to repeatedly mobilize to defend their borders. After a while, with the Egyptians continually acting as if they would invade but not doing so, the Israelis began to ignore the provocations. When Egypt finally did invade, the Israelis were caught by surprise.
Finally, the issue of habituation within the realm of substance abuse should be addressed. There is a psychological as well as physiological component to habituation when the topic turns to drug or alcohol addiction. Both mentally and physically, the addict is convinced of the need for the substance. It has become a part of his or her daily regimen, and the absence of that substance can cause psychological withdrawal symptions as potent as any physical craving.