The Collaborative International Dictionary of English v.0.48:

Thunderclap \Thun"der*clap`\, n.
   A sharp burst of thunder; a sudden report of a discharge of
   atmospheric electricity. "Thunderclaps that make them quake."
   --Spenser.
   [1913 Webster]

         When suddenly the thunderclap was heard. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thunderclap
    n 1: a single sharp crash of thunder
    2: a shocking surprise; "news of the attack came like a
       bombshell" [syn: bombshell, thunderbolt, thunderclap]