A flipper is a broad, flattened limb adapted for aquatic locomotion. It refers to the fully webbed, swimming appendages of aquatic vertebrates that are not fish.

In animals with two flippers, such as whales, the flipper refers solely to the forelimbs. In animals with four flippers, such as pinnipeds and sea turtles, one may distinguish fore- and hind-flippers, or pectoral flippers and pelvic flippers.

Animals with flippers include penguins (whose flippers are also called wings), cetaceans (e.g., dolphins and whales), pinnipeds (e.g., walruses, earless and eared seals), sirenians (e.g., manatees and dugongs), and marine reptiles such as the sea turtles and the now-extinct plesiosaurs, mosasaurs, ichthyosaurs, and metriorhynchids.

Usage of the terms "fin" and "flipper" is sometimes inconsistent, even in the scientific literature. However, the hydrodynamic control surfaces of fish are always referred to as "fins" and never "flippers". Tetrapod limbs which have evolved into fin-like structures are usually (but not always) called "flippers" rather than fins. The dorsal structure on cetaceans is called the "dorsal fin" and the large cetacean tails are referred to primarily as flukes but occasionally as "caudal fins"; neither of these structures are flippers.

Some flippers are very efficient hydrofoils, analogous to wings (airfoils), used to propel and maneuver through the water with great speed and maneuverability (see Foil). Swimming appendages with the digits still apparent, as in the webbed forefeet of amphibious turtles and platypus, are considered paddles rather than flippers.

For all species of aquatic vertebrates, swimming performance depends upon the animal's control surfaces, which include flippers, flukes and fins. Flippers are used for different types of propulsion, control, and rotation. In cetaceans, they are primarily used for control while the fluke is used for propulsion.

The evolution of flippers in penguins was at the expense of their flying capabilities, in spite of evolving from an auk-like ancestor that could 'fly' underwater as well in the air. Form constrains function, and the wings of diving flying species, such as the murre or cormorant have not developed into flippers. The flippers of penguins became thicker, denser and smaller while being modified for hydrodynamic properties.

Cetacean flippers may be viewed as being analogous to modern engineered hydrofoils, which have hydrodynamic properties: lift coefficient, drag coefficient and efficiency. Flippers are one of the principal control surfaces of cetaceans (whales, dolphins and porpoises) due to their position in front of the center of mass, and their mobility which provides three degrees of freedom.