Fin

A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilise motion while traveling in water, air, or other fluid media. Fins are also used to increase surface areas for heat transfer purposes, or simply as an ornamentation.

In fish, fins initially evolved as limbs which provided a means of locomotion. Fish fins can be used to generate thrust and control the subsequent motion. Fish, and other aquatic animals such as cetaceans, actively propel and steer themselves with pectoral and tail fins. As they swim, they can use other fins, such as dorsal and anal fins to achieve stability and refine their maneuvering.