Nautical . a vertical blade at the stern of a vessel that can be turned horizontally to change the vessel's direction when in motion.

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.

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.