Fin (noun)

1. A flattened appendage on the back of a fish or marine mammal used for propulsion and steering.
2. A similar appendage on the back of a shark, ray, or other aquatic animal.
3. A device on the side of a boat or aircraft used to stabilize movement or change direction.

Origin:

From the old english word finn.

Examples:

1. The dorsal fin of the shark was visible above the surface of the water.
2. The fins on the sides of the boat helped to keep it stable in choppy waters.
3. The fins on the aircraft helped to control its direction during flight.
4. The fish swam gracefully through the water with the help of its fins.
5. The fins of the dolphins were used to swim faster and more efficiently.