The Collaborative International Dictionary of English v.0.48:

Decadent \De*ca"dent\, a.
   Decaying; deteriorating.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decadent \De*ca"dent\, n.
   One that is decadent, or deteriorating; esp., one
   characterized by, or exhibiting, the qualities of those who
   are degenerating to a lower type; -- specif. applied to a
   certain school of modern French writers.

         The decadents and [ae]sthetes, and certain types of
         realists.                                --C. L. Dana.

         The business men of a great State allow their State to
         be represented in Congress by "decadents". --The
                                                  Century.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

decadent
    adj 1: marked by excessive self-indulgence and moral decay; "a
           decadent life of excessive money and no sense of
           responsibility"; "a group of effete self-professed
           intellectuals" [syn: decadent, effete]
    n 1: a person who has fallen into a decadent state (morally or
         artistically)