n 1  :  intellectual or moral decline tending toward dissolution of character or integrity  :  a progressive worsening of personal adjustment  2 a  :  progressive deterioration of physical characters from a level representing the norm of earlier generations or forms  :  regression of the morphology of a group or kind of organism toward a simpler less highly organized state <parasitism leads to >   b  :  deterioration of a tissue or an organ in which its vitality is diminished or its structure impaired : esp  :  deterioration in which specialized cells are replaced by less specialized cells (as in fibrosis or in malignancies) or in which cells are functionally impaired (as by deposition of abnormal matter in the tissue) .
 
   
 
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