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]