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):

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

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "decadence":
   abandon, abandonment, abjection, comedown, corruptedness,
   corruption, corruptness, debasement, decadency, decadent, decayed,
   declension, declination, decline, deformation, degeneracy,
   degenerate, degenerateness, degeneration, degradation,
   demoralization, demotion, depravation, depravedness, depravity,
   depreciation, derogation, descent, deterioration, devolution,
   dissoluteness, downfall, downgrade, downtrend, downturn,
   downward mobility, downward trend, drop, dying, ebb, effeteness,
   fading, failing, failure, failure of nerve, fall, falling-off,
   involution, lapse, loss of tone, moral pollution, moral turpitude,
   overripe, profligacy, regress, regression, reprobacy, retrocession,
   retrogradation, retrogression, rottenness, slippage, slump,
   turpitude, wane