fin

A fin is the part of a fish's body that protrudes into the water and helps it balance and move in various directions. Sharks use their dorsal fins to stabilize their bodies as they propel through the water.

Fish, whales, dolphins, and other aquatic animals have fins on various parts of their bodies. Surfboards often have a rear fin that works in a similar way, helping to keep the board steady and aid in making turns. Ships, cars, and aircraft also have fins that assist with balance. In Old English, it was finn, which probably comes from the Latin pinna, "feather or wing."