waterspout

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tornado or less violent whirlwind occurring over water, consisting of a funnel-shaped whirling column of air and water droplets.
• noun A hole or pipe from which water is discharged.

from The Century Dictionary.

• noun A pipe, nozle, or orifice from which water is spouted.
• noun A spout, jet, or column of water; specifically, a whirlwind over a body of water, producing the appearance of a solid column of water extending from the surface to the clouds.

from the GNU version of the Collaborative International Dictionary of English.

• noun A remarkable meteorological phenomenon, of the nature of a tornado or whirlwind, usually observed over the sea, but sometimes over the land.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a tornado that occurs over a body of water
• noun a channel through which water is discharged, especially from the gutters of a roof

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a tornado passing over water and picking up a column of water and mist
• noun a channel through which water is discharged (especially one used for drainage from the gutters of a roof)
• noun a heavy rain