The Collaborative International Dictionary of English v.0.48:

Explosion \Ex*plo"sion\, n. [L. explosio a driving off by
   clapping: cf. F. explosion explosion. See Explode.]
   1. The act of exploding; detonation; a chemical action which
      causes the sudden formation of a great volume of expanded
      gas; as, the explosion of gunpowder, of fire damp, etc.
      [1913 Webster]

   2. A bursting with violence and loud noise, because of
      internal pressure; as, the explosion of a gun, a bomb, a
      steam boiler, etc.
      [1913 Webster]

   3. A violent outburst of feeling, manifested by excited
      language, action, etc.; as, an explosion of wrath.
      [1913 Webster]

            A formidable explosion of high-church fanaticism.
                                                  --Macaulay.
      [1913 Webster]

   4. a sudden and substantial increase; a rapid acceleration;
      as, the population explosion.
      [PJC]

WordNet (r) 3.0 (2006):

explosion
    n 1: a violent release of energy caused by a chemical or nuclear
         reaction [syn: explosion, detonation, blowup]
    2: the act of exploding or bursting; "the explosion of the
       firecrackers awoke the children"; "the burst of an atom bomb
       creates enormous radiation aloft" [syn: explosion, burst]
    3: a sudden great increase; "the population explosion"; "the
       information explosion"
    4: the noise caused by an explosion; "the explosion was heard a
       mile away"
    5: the terminal forced release of pressure built up during the
       occlusive phase of a stop consonant [syn: plosion,
       explosion]
    6: a sudden outburst; "an explosion of laughter"; "an explosion
       of rage"
    7: a golf shot from a bunker that typically moves sand as well
       as the golf ball

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "explosion":
   accelerando, acceleration, access, aggravation, backfire, bang,
   bark, beefing-up, blast, blaze, blaze of temper, blowing up,
   blowout, blowup, boom, burgeoning, burst, clap, concentration,
   condensation, consolidation, convulsion, crack, crash, cyclone,
   deepening, detonation, discharge, disconfirmation, disproof,
   disproval, disproving, enhancement, eruption, exacerbation,
   exaggeration, expansion, expose, exposure, fit, flare, flare-up,
   flash, fulguration, fulmination, fusillade, gale, gunshot, gust,
   heating-up, heightening, high words, hurricane, increase,
   information explosion, intensification, invalidation, irruption,
   magnification, mushrooming, negation, outbreak, outburst, paddy,
   paroxysm, pickup, pop, population explosion, redargution,
   redoubling, reductio ad absurdum, reinforcement, report, sally,
   salvo, scene, seizure, shot, spasm, speedup, step-up, storm,
   strengthening, tantrum, tempest, tightening, tornado, upheaval,
   volley, wax, whirlwind