A fin is an organ or device used to control movement in an aquatic or other liquid medium. It is most often flat in shape. The fins can be paired (thoracic and ventral) and unpaired (dorsal, anal, caudal).

Pectoral fins are paired fins that serve as rudders, balancers, brakes, supporting planes, and sometimes organs of motion (rays); they are absent in roundfishes, eels, and some sea urchins. Pectoral fins vary in shape and size. They are an important systematic feature.

The paired pectoral fins are located on each side, usually kept folded just behind the operculum, and are homologous to the forelimbs of tetrapods. A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "flight" for flying fish. 

In many fish, the pectoral fins aid in walking, especially in the lobe-like fins of some anglerfish and in the mudskipper. Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards. In skates and rays, the pectoral fins are used for propulsion (rajiform propulsion). The "horns" of manta rays and their relatives are called cephalic fins; this is actually a modification of the anterior portion of the pectoral fin.

The pectoral fin at its base has one row of small bones, radialia, departing from the scapus (partially from coracoid as well). The whole free fin blade consists of lepidotrichia. A peculiarity of the skeleton of pectoral fins of bony fish, as compared to cartilaginous ones, is the reduction of the basalii. The mobility of the pectoral fins is increased because the muscles are attached to the expanded bases of the dermal rays movably articulating with the radials.

According to the presence and nature of the rays in the fins of most bony fish, a fin formula is compiled, which is widely used in their description and definition. In this formula, Latin letters are the abbreviated name of the fin: A - anal fin, P - pectoral fin, V - pelvic fin, D - dorsal fin; Roman numerals give the number of barbed, and Arabic - soft rays.