The Collaborative International Dictionary of English v.0.48:

Cloud-burst \Cloud"-burst`\, n.
   A sudden copious rainfall, as if the whole cloud had been
   precipitated at once.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cloudburst
    n 1: a heavy rain [syn: downpour, cloudburst, deluge,
         waterspout, torrent, pelter, soaker]