The Collaborative International Dictionary of English v.0.48:

Thunderbolt \Thun"der*bolt`\, n.
   1. A shaft of lightning; a brilliant stream of electricity
      passing from one part of the heavens to another, or from
      the clouds to the earth.
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   2. Something resembling lightning in suddenness and
      effectiveness.
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            The Scipios' worth, those thunderbolts of war.
                                                  --Dryden.
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   3. Vehement threatening or censure; especially,
      ecclesiastical denunciation; fulmination.
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            He severely threatens such with the thunderbolt of
            excommunication.                      --Hakewill.
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   4. (Paleon.) A belemnite, or thunderstone.
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   Thunderbolt beetle (Zool.), a long-horned beetle
      (Arhopalus fulminans) whose larva bores in the trunk of
      oak and chestnut trees. It is brownish and bluish-black,
      with W-shaped whitish or silvery markings on the elytra.
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WordNet (r) 3.0 (2006):

thunderbolt
    n 1: a discharge of lightning accompanied by thunder [syn:
         thunderbolt, bolt, bolt of lightning]
    2: a shocking surprise; "news of the attack came like a
       bombshell" [syn: bombshell, thunderbolt, thunderclap]