down-time

from The Free On-line Dictionary of Computing (8 July 2008)

down-time

   <jargon> A period of time during which a (computer) system is
   not operational, due to a malfunction or maintenance.

   (1997-06-08)

from Moby Thesaurus II by Grady Ward, 1.0

44 Moby Thesaurus words for "downtime":
      break, breath, breather, breathing place, breathing space,
      breathing spell, breathing time, cigarette break, cocktail hour,
      coffee break, enforced respite, half time, half-time intermission,
      halt, happy hour, holiday, interim, interlude, intermezzo,
      intermission, intermittence, interregnum, interruption, interval,
      letup, lull, off-time, pause, plateau, point of repose,
      quiet spell, recess, relief, respite, rest, resting point, spell,
      stay, surcease, suspension, tea break, time off, time out,
      vacation