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