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: Webster's Revised Unabridged Dictionary (1913)