Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

tornado
     n 1: a localized and violently destructive windstorm occurring
          over land characterized by a funnel-shaped cloud
          extending toward the ground [syn: {twister}]
     2: a purified and potent form of cocaine that is smoked rather
        than snorted [syn: {crack}]
     [also: {tornadoes} (pl)]

tornadoes
     See {tornado}