Definitions
1.nounAn underwater vane used to steer a vessel. The rudder is controlled by means of a wheel, tiller or other apparatus (modern vessels can be controlled even with a joystick or an autopilot).
2.nounA control surface on the vertical stabilizer of a fixed-wing aircraft or an autogyro. On some craft, the entire vertical stabilizer comprises the rudder. The rudder is controlled by foot-operated control pedals.
3.nounA riddle or sieve.
4.nounThat which resembles a rudder as a guide or governor; that which guides or governs the course.