The Collaborative International Dictionary of English v.0.48:

Downpour \Down"pour`\, n.
   A pouring or streaming downwards; esp., a heavy or continuous
   shower.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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