1. Changes in cells, tissues, or organs due to disease, etc., that result in an impairment or loss of function and possibly death and breakdown of the affected part.

2. The reduction in size or complete loss of organs during evolution. The human appendix has undergone this process and performs no obvious function in humans. Degeneration of external organs may cause animals to appear to be more ‘primitive’ than they really are; for example, early zoologists believed whales were fish rather than mammals because of the degeneration of their limbs. See also vestigial organ.