A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, (in other words, a foil (fluid mechanics)). The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices. Fins, as with other foils, operate in fluids such as water or air.
Fins are seen both in nature and in manmade iterations.
Swimming water animals such as fish and cetaceans actively use pectoral fins for maneuvering, and dorsal fins contribute stability as the animal swims, propelling and maneuvering with its tail, itself recognizable as a fin.
The fin on fixed-wing aircraft is known as a vertical stabilizer. Fins are also seen used as e.g., fletching on arrows and at the rear of some bombs, missiles, rockets, and self-propelled torpedoes. These are typically "planar" (shaped like small wings), although grid fins are sometimes used in specialized cases.
Examples of fins include: