The Collaborative International Dictionary of English v.0.48:

Tornado \Tor*na"do\, n.; pl. Tornadoes. [From Sp. or Pg.
   tornar to turn, return, L. tornare to turn, hence, a whirling
   wind. The Sp. & Pg. tornada is a return. See Turn.]
   A violent whirling wind; specifically (Meteorol.), a tempest
   distinguished by a rapid whirling and slow progressive
   motion, usually accompaned with severe thunder, lightning,
   and torrents of rain, and commonly of short duration and
   small breadth; a small cyclone.
   [1913 Webster]