1. The process of habituating or the state of being habituated.
2. 1. Physiological tolerance to a drug resulting from repeated use.
   2. Psychological dependence on a drug.
3. Psychology. The decline of a conditioned response following repeated exposure to the conditioned stimulus.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.