paddle wheel

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

• noun A wheel with boards or paddles affixed around its circumference, usually driven by steam to propel a ship.

from The Century Dictionary.

• noun A wheel (generally one of two placed at the sides of a steam-vessel) provided with boards or floats on its circumference, and driven by steam, for the propulsion of the vessel.
• noun A wheel fitted with paddles, used to aid, by its revolution, in certain washing operations, as in leather-manufacture, etc.

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

• noun nautical A large wheel, fitted with paddles, used to propel a vessel (especially a paddle steamer)

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

• noun a large wheel fitted with paddles and driven by an engine in order to propel a boat
• noun a large wheel fitted with paddles and driven by an engine in order to propel a boat