The Collaborative International Dictionary of English v.0.48:

outbreak \out"break`\, n.
   1. A bursting forth; eruption; insurrection; mutiny; revolt.
      "Mobs and outbreaks." --J. H. Newman.
      [1913 Webster]

            The flash and outbreak of a fiery mind. --Shak.
      [1913 Webster]

   2. A sudden beginning of a violent event; as, the outbreak of
      hostilities between ethnic groups.
      [PJC]

   3. A sudden occurrence or manifestation; -- usually of
      disease or emotion, in one person or a group; as, an
      outbreak of measles among the students; he had an outbreak
      of shingles; an outbreak of nervousness in the mob.
      [PJC]

WordNet (r) 3.0 (2006):

outbreak
    n 1: a sudden violent spontaneous occurrence (usually of some
         undesirable condition); "the outbreak of hostilities" [syn:
         outbreak, eruption, irruption]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "outbreak":
   A, alpha, beginning, belch, blast-off, blaze, burst, cascade,
   civil disorder, commencement, convulsion, creation, cutting edge,
   cyclone, dawn, debouchment, dissiliency, edge, emeute, eructation,
   eruption, establishment, explosion, fit, flare-up, flying start,
   foundation, fresh start, gale, general uprising, gush, gust,
   hurricane, institution, insurgence, insurgency, insurrection,
   irruption, jacquerie, jet, jump-off, kick-off, leading edge,
   levee en masse, mutiny, new departure, oncoming, onset, opening,
   origin, origination, outburst, outset, paroxysm, peasant revolt,
   putsch, rapids, rebellion, revolt, revolution, riot, rising,
   running start, rush, seizure, send-off, setting in motion,
   setting-up, spasm, spate, spew, spurt, square one, start,
   start-off, starting point, storm, take-off, tempest, tornado,
   torrent, upheaval, uprising, volcan, whirlwind