Flipper

A flipper is a flattened limb or appendage adapted to propel or steer an animal in water. It occurs in several groups of aquatic vertebrates and varies in structure and function.

Hydrodynamics: The study of fluids in motion and the forces they exert. Fluid forces explain how flipper shape and movement produce swimming force.
Tetrapod limb: A vertebrate limb descended from the paired appendages of four-limbed ancestors. Many mammalian flippers are modified tetrapod limbs.
Cetacean flipper: The flattened forelimb of a whale, dolphin, or porpoise, used chiefly for steering and stability. Cetaceans retain forelimb bones inside a streamlined external paddle.
Wing: An animal appendage adapted primarily to generate lift for flight. Penguin flippers are evolutionarily wings, though they propel the animal underwater.
Secondary aquatic adaptation: The evolution of aquatic traits in lineages whose ancestors lived on land. Mammalian flippers arose as terrestrial limbs were remodeled for aquatic life.
Drag: A force that opposes an object's motion through a fluid or gas. Flipper shape balances thrust against resistance as it moves through water.
Homology (biology): Similarity between biological structures due to inheritance from a common ancestor. Shared bone patterns identify flippers as modified versions of ancestral limbs.
Pinniped flipper: A limb of a seal, sea lion, or walrus adapted for swimming and, in some species, land movement. Pinnipeds use their forelimbs or hind limbs differently across swimming styles.
Paddle: A broad blade moved through water to propel or steer a vessel or object. A paddle resembles a flipper in shape but is not an animal limb.
Pachyostosis: The abnormal thickening and increased density of bones, common in some aquatic vertebrates. Dense bones in some aquatic animals affect buoyancy and flipper use.

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