WordNet (r) 3.0 (2006):

downtime
    n 1: a period of time when something (as a machine or factory)
         is not operating (especially as a result of malfunctions)
         [ant: uptime]

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