2. The fact of being extinct or the process of becoming extinct: the extinction of the passenger pigeon; languages that are in danger of extinction.

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.

5. Astronomy The dimming of celestial objects, usually measured in magnitudes, due to scattering and absorption of their light as it passes through interstellar dust clouds and the earth's atmosphere.