NOUN (2)

1. (meteorology) rapid inward circulation of air masses about a low pressure center; circling counterclockwise in the northern hemisphere and clockwise in the southern;

2. a violent rotating windstorm;

The Collaborative International Dictionary of English v.0.48:

Cyclone \Cy"clone\ (s?"kl?n), n. [Gr.????? moving in a circle,
   p. pr. of ?????, fr. ky`klos circle.]
   1. (Meteor.) A violent storm, often of vast extent,
      characterized by high winds rotating about a calm center
      of low atmospheric pressure. This center moves onward,
      often with a velocity of twenty or thirty miles an hour.
      [1913 Webster]

   Note: The atmospheric disturbance usually accompanying a
         cyclone, marked by an onward moving area of high
         pressure, is called an anticyclone.
         [1913 Webster]

   2. (Meteor.) In general, a condition of the atmosphere
      characterized by a central area of pressure much lower
      than that of surrounding areas, and a system of winds
      blowing inward and around (clockwise in the southern
      hemisphere and counter-clockwise in the northern); --
      called also a low-area storm. It is attended by high
      temperature, moist air, abundant precipitation, and
      clouded sky. The term includes the hurricane, typhoon, and
      tropical storms; it should not be applied to the moderate
      disturbances attending ordinary areas of low pressure nor
      to tornadoes, waterspouts, or "twisters," in which the
      vertical motion is more important than the horizontal.
      [Webster 1913 Suppl.]

   3. A tornado. See above, and Tornado. [Middle U. S.]
      [Webster 1913 Suppl.] Cyclone cellar

WordNet (r) 3.0 (2006):

cyclone
    n 1: (meteorology) rapid inward circulation of air masses about
         a low pressure center; circling counterclockwise in the
         northern hemisphere and clockwise in the southern [ant:
         anticyclone]
    2: a violent rotating windstorm

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "cyclone":
   air mass, anticyclone, baguio, blaze, burst, cold front,
   cold sector, convulsion, dirty weather, dust devil, eruption,
   explosion, fit, flare-up, foul weather, front, gale, gust, high,
   high-pressure area, hurricane, irruption, isobar, isometric,
   isometric line, isopiestic line, isotherm, isothermal line,
   line squall, low, low-pressure area, occluded front, outbreak,
   outburst, paroxysm, polar front, rainspout, rainstorm,
   rotary storm, rough weather, sand column, sandspout, seizure,
   snowstorm, spasm, squall, squall line, stationary front, storm,
   stormy weather, tempest, tempestuous rage, thunderstorm, tornado,
   twister, typhoon, upheaval, warm front, waterspout, weather map,
   whirlblast, whirlwind, wind eddy, wind-shift line, windstorm