The Collaborative International Dictionary of English v.0.48:

Outburst \Out"burst`\, n.
   A bursting forth; as, an outburst of laughter.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

outburst
    n 1: an unrestrained expression of emotion [syn: effusion,
         gush, outburst, blowup, ebullition]
    2: a sudden intense happening; "an outburst of heavy rain"; "a
       burst of lightning" [syn: outburst, burst, flare-up]
    3: a sudden violent disturbance [syn: outburst, tumultuous
       disturbance]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "outburst":
   access, attack, belch, blaze, blaze of temper, blowout, blowup,
   blurt, burst, cascade, convulsion, cyclone, debouchment,
   deflagration, discharge, disgorgement, dissiliency, ecphonesis,
   effluence, efflux, effusion, ejaculation, emission, eructation,
   eruption, exclamation, expletive, explosion, expulsion,
   extravasation, fit, flare, flare-up, flash, flash fire, flood,
   fulmination, gale, gush, gust, high words, hurricane, irruption,
   jet, outbreak, outflowing, outpour, outpouring, paroxysm, rapids,
   rush, scene, seizure, spasm, spate, spew, spout, spurt, squirt,
   storm, surge, tantrum, tempest, tornado, torrent, upheaval,
   upsurge, volcan, whirlwind