A fin is a thin, flat body part that fish and some other water animals use to move through water and steer. If you've ever watched a goldfish swim, you've seen its fins waving gracefully. The large fin on a fish's back is called the dorsal fin, the one on its tail is the tail fin, and the smaller ones on its sides help it balance and turn.

Sharks have famously triangular dorsal fins that slice through the water's surface. Dolphins and whales, which are mammals rather than fish, also have fins. Some water-dwelling reptiles like sea turtles have flippers, which work like fins but are structured differently.

The word fin also appears in other contexts. Divers wear rubber fins on their feet to help them swim more powerfully underwater, copying how fish use their natural fins. Engineers sometimes attach fins to cars, surfboards, or even spacecraft to help control movement and direction. In the 1950s, American cars had dramatic tail fins purely for style, inspired by rocket ships and jets.