Plain English Meaning
The hinged flap on the back of the tail fin that the pilot moves with the foot pedals to swing the nose of the aircraft left or right.
Full Definition
A movable, vertical control surface hinged to the trailing edge of the vertical stabilizer (fin) at the tail of an aircraft. The rudder is controlled by the pilot's foot pedals and is used to rotate the aircraft about its vertical (yaw) axis, swinging the nose left or right.
Context Anchor
Pilots use the rudder through the foot pedals during takeoff, landing, turns, and any time the airplane needs left-right nose .