Definition
The fixed vertical surface at the rear of an aircraft that provides directional (yaw) stability. The movable rudder is hinged to the trailing edge of the fin.
Plain English
The fin is the upright, non-moving part of the tail. It keeps the aircraft pointed straight in flight, like the feathers on an arrow. The rudder is attached to the back of it.
Context Anchor
Seen in flight control system discussions, especially when describing the rudder and the tail surfaces of an airplane.
Derivation
From Old English 'finn,' the stiff projection on a fish that helps it track straight through water. Aircraft designers borrowed the word because the tail surface does the same job in air — it stops the aircraft from swinging side to side.
Why Pilots Care
Loss of the fin's stabilizing effect leads to uncontrolled yaw and loss of directional control.
Intuition Check
Do not think of the fin as the moving control. In this context, the fin is the fixed upright tail surface; the rudder is the part that moves.
Example Sentence 1
During the preflight walkaround, the pilot checked the fin and rudder for any signs of damage or loose fasteners.
Example Sentence 2
Damage to the fin during flight can make the airplane difficult to keep aligned with the runway.