Fish fin

A flexible appendage of a fish used for propulsion, steering, balance, or stabilization. Fins can be supported by bony or cartilaginous rays, or form fleshy lobes.

Fin ray: A slender skeletal support within a fish fin, made of bone or cartilage. Rays stiffen flexible fin membranes while allowing them to bend during movement.
Fish locomotion: The movement of fish through water using body undulations, fins, or both. Fins operate as part of a swimming system that includes the body and tail.
Labriform locomotion: A swimming mode in which oscillating pectoral fins provide most propulsion. Wrasses and related fishes use paired fins for maneuvering without strong body undulation.
Paired appendage evolution: The evolutionary history of paired fins and limbs in vertebrates. Fish fins are central to explaining how paired appendages arose and diversified.
Ray-finned fish: A fish in the class Actinopterygii, whose fins are supported mainly by bony rays. Most living fish belong to this group, whose fins typically have thin, ray-supported membranes.
Fin membrane: The thin tissue stretched between supports in a fish fin. Its shape and flexibility determine how the fin pushes against water.
Thrust: A force that propels an object forward through a fluid or other medium. Fin strokes create thrust by accelerating water backward.
Amiiform locomotion: A swimming mode driven by undulations of a long dorsal fin. Bowfin demonstrate how a dorsal fin can propel a fish independently of its tail.
Panderichthys: An extinct Devonian fish with limb-like features in its paired fins and skull. Its pectoral skeleton shows an intermediate arrangement near the fish–tetrapod transition.
Lobe-finned fish: A fish in the class Sarcopterygii, with paired fins supported by fleshy lobes and internal bones. Its muscular, internally supported fins differ from the prominent rays of most fishes.

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