The Collaborative International Dictionary of English v.0.48:

Thunderstorm \Thun"der*storm`\, n.
   A storm accompanied with lightning and thunder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thunderstorm
    n 1: a storm resulting from strong rising air currents; heavy
         rain or hail along with thunder and lightning [syn:
         thunderstorm, electrical storm, electric storm]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "thunderstorm":
   Donar, Indra, Jupiter Tonans, Thor, cyclone, deep, dirty weather,
   dread rattling thunder, foul weather, hurricane, line squall,
   peal of thunder, rainstorm, rough weather, snowstorm, squall,
   storm, stormy weather, tempest, tempestuous rage, thunder,
   thunderclap, thundercrack, thundering, thunderpeal, thundershower,
   thundersquall, thunderstroke, tornado, typhoon, windstorm