Fins are an exceedingly common form of biological locomotive adaptation. They are thin appendages attached to a larger body or structure. They typically function as foils that produce lift, or thrust or provide the ability to steer or stabilize motion while traveling through water or something analogous. The fins on the tails of cetaceans, mosasaurs, etc., are called Flukes.

Fins are some of the most common forms of biological locomotion known to the Prime Material Plane, being present on almost any aquatic organism or organism which moves through something analogous to water. They are even found on certain terrestrial organisms, albeit in most cases atrophied.