WordWeb Online

1. Changing for the worse
   "worried by the worsening storm"

1. Process of changing to an inferior state
   - deterioration, decline in quality, declension
    
2. Changing something with the result that it becomes worse

1. Grow worse
   "Conditions in the slum worsened"
   - decline
    
2. Make worse
   "This drug worsens the pain"
   - aggravate, exacerbate, exasperate

Type of: alter, change, change of state, change state, decline, diminution, modify, turn

Antonym: better, bettering