The Collaborative International Dictionary of English v.0.48:

Decadence \De*ca"dence\, Decadency \De*ca"den*cy\, n. [LL.
   decadentia; L. de- + cadere to fall: cf. F. d['e]cadence. See
   Decay.]
   A falling away; decay; deterioration; declension. "The old
   castle, where the family lived in their decadence." --Sir W.
   Scott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decadency
    n 1: the state of being degenerate in mental or moral qualities
         [syn: degeneracy, degeneration, decadence,
         decadency]