Noun

1. The act or state of growing worse, or the state of having become worse; decline; degradation; debasement; degeneracy; deterioration.
   • 1913, B. H. Carrol, An Interpretation of the English Bible,

     The modern cry of "more liberty and less creed" is a degeneration from a vertebrate to a jellyfish.

2. That condition of a tissue or an organ in which its vitality has become either diminished or perverted; a substitution of a lower for a higher form of structure.

   fatty degeneration of the liver

3. A gradual deterioration, from natural causes, of any class of animals or plants or any particular organ or organs; hereditary degradation of type.
4. The thing degenerated.