[ruhd-er] 
| 1. | Nautical. a vertical blade at the stern of a vessel that can be turned horizontally to change the vessel's direction when in motion. |
| 2. | Aeronautics. a movable control surface attached to a vertical stabilizer, located at the rear of an airplane and used, along with the ailerons, to turn the airplane. |
| 3. | any means of or device for governing, directing, or guiding a course, as a leader or principle: His ideas provided a rudder for the new company. |
| rud·der (rŭd'ər) n.
[Middle English ruder, from Old English rōther, steering oar; see erə- in Indo-European roots.] |
rudder
part of the steering apparatus of a boat or ship that is fastened outside the hull, usually at the stern. The most common form consists of a nearly flat, smooth surface of wood or metal hinged at its forward edge to the sternpost. It operates on the principle of unequal water pressures. When the rudder is turned so that one side is more exposed to the force of the water flowing past it than the other side, the stern will be thrust away from the side that the rudder is on and the boat will swerve from its original course. In small craft the rudder is operated manually by a handle termed a tiller or helm. In larger vessels, the rudder is turned by hydraulic, steam, or electrical machinery.
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