Source : WordNet®

downtime
     n : a period of time when something (as a machine or factory) is
         not operating (especially as a result of malfunctions)
         [ant: {uptime}]

Source : Free On-Line Dictionary of Computing

down-time
     
         A period of time during which a (computer) system is
        not operational, due to a malfunction or maintenance.
     
        (1997-06-08)