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]