Animal fin

An animal fin is a flexible or rigid body appendage that helps an animal swim, steer, stabilize itself, or display. Fins evolved in several animal groups and vary widely in structure and function.

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Hydrodynamics: The study of fluids in motion and their forces on objects. Water resistance and flow around a fin shape its swimming performance.
Pectoral fin: A paired fin attached near the pectoral region of a fish or other aquatic vertebrate. Pectoral fins commonly steer, brake, and provide lift.
Fish fin: A fish appendage supported by fin rays, cartilage, or other internal structures. Fish fins are the familiar reference point, but animal fins are not all fish fins.
Fish locomotion: The ways fishes move through water using body and fin motions. Fin use varies with swimming speed, maneuvering, and habitat.
Fin-to-limb transition: The evolutionary transformation of paired fish fins into the limbs of tetrapods. Fossils and development connect fin structure to the ancestry of limbs.
Lift: A force perpendicular to fluid flow, produced by pressure differences around a moving surface. Angled fins create lift that can turn or stabilize a swimmer.
Pelvic fin: A paired fin attached to the pelvic region of a fish. Its position and movement can help stabilize or maneuver the body.
Cetacean flipper: A whale or dolphin forelimb modified into a flattened swimming appendage. A flipper is a limb with bones, unlike a typical fish fin.
Fish schooling: Coordinated group swimming by fishes, often with aligned direction and spacing. Fin control supports precise spacing and rapid turns within schools.
Tiktaalik: An extinct Devonian vertebrate with fish features and limb-like bones in its paired fins. Its fin skeleton shows an intermediate arrangement relevant to limb evolution.