1. 1. The act of extinguishing.
   2. The condition of being extinguished.
2. The fact of being extinct or the process of becoming extinct: "The most effective agent in the extinction of species is the pressure of other species" (Alfred R. Wallace).
3. Psychology. A reduction or a loss in the strength or rate of a conditioned response when the unconditioned stimulus or reinforcement is withheld.
4. Physiology. A gradual decrease in the excitability of a nerve to a previously adequate stimulus, usually resulting in total loss of excitability.

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.