The Collaborative International Dictionary of English v.0.48:

Deterioration \De*te`ri*o*ra"tion\, n. [LL. deterioratio: cf. F.
   d['e]t['e]rioration.]
   The process of growing worse, or the state of having grown
   worse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deterioration
    n 1: a symptom of reduced quality or strength [syn:
         deterioration, impairment]
    2: process of changing to an inferior state [syn:
       deterioration, decline in quality, declension,
       worsening]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "deterioration":
   about-face, accommodation, adaptation, adjustment, aggravation,
   alteration, amelioration, amplification, annoyance, apostasy,
   atrophy, augmentation, betterment, break, change, change of heart,
   changeableness, comedown, constructive change, contentiousness,
   continuity, conversion, crumbling, debasement, decadence,
   decadency, decay, decaying, declension, declination, decline,
   decomposition, deepening, defection, deformation, degeneracy,
   degenerateness, degeneration, degenerative change, degradation,
   deliberate aggravation, demotion, depravation, depravedness,
   depreciation, derogation, descent, devaluation, deviation,
   devolution, difference, discontinuity, disintegration, dislocation,
   disruption, dissolution, divergence, diversification, diversion,
   diversity, downfall, downgrade, downtrend, downturn,
   downward mobility, downward trend, drop, dry rot, dying, ebb,
   ebbing, effeteness, embittering, embitterment, enhancement,
   enlargement, exacerbation, exasperation, fading, failing, failure,
   failure of nerve, fall, falling-off, fitting, flip-flop,
   gradual change, heightening, impairment, improvement, increase,
   intensification, involution, irritation, lapse, lessening,
   loss of tone, magnification, melioration, mitigation, modification,
   modulation, overthrow, provocation, qualification, radical change,
   re-creation, realignment, redesign, reform, reformation,
   regression, remaking, renewal, reshaping, restructuring,
   retrocession, retrogradation, retrogression, reversal, revival,
   revivification, revolution, rotting, ruin, sharpening, shift,
   slippage, slump, souring, spoiling, sudden change, switch,
   total change, transition, turn, turnabout, upheaval, variation,
   variety, violent change, wane, waning, worsening