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organ of locomotion characteristic of fish and consisting of thin tissue supported by cartilaginous or bony rays. In some fish, e.g., the eel, a single fin extends from the back, around the tail, and along the ventral surface. In the majority of fishes, however, there are one, two, or three dorsal fins, a distinct tail fin, and an anal fin. These are called median, or unpaired, fins. In addition to these unpaired fins, most fish also have paired fins. These are the pectoral fins, placed just back of the gills, and the pelvic, or ventral, fins, variable in position and sometimes lacking entirely. The tail is an important organ of locomotion and the paired fins are used for steering, checking speed, balancing, and for slow movements. An adipose fin (fatty tissue without support) is found behind the dorsal fin in some fish, e.g., the salmon and the catfish. See climbing perchclimbing perch,
 climbing gourami,
or walking fish,
any of several fish of the family Anabantidae, adapted to living in oxygen-depleted water or on dry land. They are not related to the true perch.
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; flying fishflying fish,
common name for members of the Exocoetidae, a family of carnivorous or herbivorous fish of warmer seas. Flying fishes usually swim in schools. They average 7 to 12 in. (17.
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an organ of locomotion of aquatic animals. Invertebrates having fins include pelagic gastropods and cephalopods, and arrowworms (Chaetognatha). The fin of a gastropod is a modified foot; in cephalopods lateral folds of skin effect locomotion. Arrowworms have lateral and caudal fins, which are formed by folds of skin.

Among extant vertebrates, fins are found in cyclostomes, fishes, some amphibians, and mammals. Cyclostomes have unpaired fins: an anterior and a posterior dorsal fin (in lampreys) and a caudal fin.

Fishes may have paired or unpaired fins. The paired fins may be anterior (pectoral) or posterior (pelvic). In some fishes, such as the Gadidae and Blenniformes, the pelvic fins are anterior to the pectoral fins. The skeleton of paired fins consists of cartilaginous or bony rays, which articulate to the skeleton of the girdles of the limbs. The principal function of paired fins is to direct the fish’s movement vertically. In a number of fishes the paired fins perform the functions of swimming, gliding in the air (in flying fishes), crawling on the bottom, or moving on dry land (in fishes that periodically emerge from the water). For example, fishes of the tropical genus Periophtalmus use their pectoral fins to climb trees. The skeleton of an unpaired fin—a dorsal fin (often divided in two or, sometimes, three parts), an anal fin (sometimes divided into two parts), or a caudal fin—consists of cartilaginous or bony rays that lie between the lateral muscles of the body. The skeletal rays of the caudal fin are connected to the posterior end of the spine; in some fishes the rays are replaced by spinal processes of the vertebrae.

The peripheral parts of the fin are supported by slender rays of horny or bony tissue. In acanthopterygian fishes, the anterior rays become thickened and form hard spines that are sometimes connected to poison glands. To the bases of these rays are attached the muscles that expand the lobe of the fin. The dorsal and anal fins serve to regulate the direction of the fish’s motion. In addition, they sometimes may serve as organs of progressive locomotion or may perform such functions as attracting prey. The caudal fin, which varies greatly in shape in various fishes, is the principal organ of locomotion.

During vertebrate evolution the fins of fishes probably arose from an unbroken fold of skin that passed along the back of the animal, bent around the posterior end of its body, continued on the ventral side to the anal opening, and then divided into two lateral folds that extended to the gill slits. Such is the location of the fin folds in the lancelet, an extant primitive chordate. It may be presumed that in the process of animal evolution, skeletal elements formed in some parts of the folds, but in the interstices the folds disappeared, leading to the development of unpaired fins in cyclostomes and fishes and of paired fins in fishes. This presumption seems to be supported by the existence of lateral folds or rows of spines in the oldest vertebrates (some Agnatha and Acanthodii) and by the fact that in extant fishes paired fins are of greater length in the early stages of development than in the adult state.

Among amphibians, unpaired fins in the form of a cutaneous fold lacking a skeleton are found as permanent or temporary formations in most aquatic larvae, adult caudates, and larvae of acaudate amphibians. Among mammals, fins are present in cetaceans and sirenians, who transferred to aquatic life. The unpaired fins of cetaceans (the vertical dorsal fin and the horizontal caudal fin) and sirenians (the horizontal caudal fin) have no skeleton. They are secondary formations and are not homologous to the unpaired fins of fishes. The paired fins of cetaceans and sirenians, consisting only of anterior fins (the posterior ones are reduced), have an internal skeleton and are homologous to the anterior limbs of all other vertebrates.


Rukovodstvo po zoologii, vol. 2. Moscow-Leningrad, 1940.
Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Suvorov, E. K. Osnovy ikhtiologii, 2nd ed. Moscow, 1947.
Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Aleev, Iu. G. Funktsional’nye osnovy vneshnego stroeniia ryby. Moscow, 1963.



(aerospace engineering)
A fixed or adjustable vane or airfoil affixed longitudinally to an aerodynamically or ballistically designed body for stabilizing purposes.
(design engineering)
A projecting flat plate or structure, as a cooling fin.
Material which remains in the holes of a molded part and which must be removed.
(vertebrate zoology)
A paddle-shaped appendage on fish and other aquatic animals that is used for propulsion, balance, and guidance.


1. An extended surface used to increase the heat transfer area, as metal sheets attached to tubes.
2. A thin flange projecting outward from the periphery of the frame of an aluminum window to serve as a means of securing the frame in a wood or masonry opening.
3. A narrow linear projection on a formed concrete surface, resulting from mortar flowing out between spaces in the formwork.
4. A thin projection on a casting or forging resulting from trimming or from the metal under pressure being forced into hairline cracks in the die or around die inserts.
5. A steel sheeting wall which projects from a main coffer-dam structure.


On drawings, abbr. for finish.


i. A fixed vertical, inclined, or adjustable airfoil or vane attached longitudinally to the rear portion of an airplane to provide a stabilizing effect.
ii. A projecting flat plate or structure, such as a cooling fin.


1. any of the firm appendages that are the organs of locomotion and balance in fishes and some other aquatic animals. Most fishes have paired and unpaired fins, the former corresponding to the limbs of higher vertebrates
a. Brit a vertical surface to which the rudder is attached, usually placed at the rear of an aeroplane to give stability about the vertical axis
b. a tail surface fixed to a rocket or missile to give stability
3. Nautical a fixed or adjustable blade projecting under water from the hull of a vessel to give it stability or control
4. a projecting rib to dissipate heat from the surface of an engine cylinder, motor casing, or radiator
5. another name for flipper


A raised source-to-drain channel in a transistor. See FinFET and transistor.
References in classic literature ?
In connexion with this appellative of Whalebone whales , it is of great importance to mention, that however such a nomenclature may be convenient in facilitating allusions to some kind of whales, yet it is in vain to attempt a clear classification of the Leviathan, founded upon either his baleen, or hump, or fin, or teeth; notwithstanding that those marked parts or features very obviously seem better adapted to afford the basis for a regular system of Cetology than any other detached bodily distinctions, which the whale, in his kinds, presents.
She still lives about the Quarter, and, though there is always a soldat, she has become a blanchisseuse de fin.
He spake no dream; for, as his words had end, Our Saviour, lifting up his eyes, beheld, In ample space under the broadest shade, A table richly spread in regal mode, With dishes piled and meats of noblest sort And savour--beasts of chase, or fowl of game, In pastry built, or from the spit, or boiled, Grisamber-steamed; all fish, from sea or shore, Freshet or purling brook, of shell or fin, And exquisitest name, for which was drained Pontus, and Lucrine bay, and Afric coast.
et eut--and had) E-u fin riche; which signifies that after first belonging to the administration, he gave it up and got rich elsewhere.
This same fish has the power of firmly catching hold of any object, such as the blade of an oar or the fishing- line, with the strong spine both of its pectoral and dorsal fin.
And I shall know that I must die, at sea most likely, cease crawling of myself to be all a-crawl with the corruption of the sea; to be fed upon, to be carrion, to yield up all the strength and movement of my muscles that it may become strength and movement in fin and scale and the guts of fishes.
Will you take fin champagne, Prince, or Chartreuse?
One voyager in Purchas calls them the wondrous whiskers inside of the whale's mouth; another, hogs' bristles; a third old gentleman in Hackluyt uses the following elegant language: There are about two hundred and fifty fins growing on each side of his upper chop, which arch over his tongue on each side of his mouth.
During their games, their bounds, while rivalling each other in beauty, brightness, and velocity, I distinguished the green labre; the banded mullet, marked by a double line of black; the round-tailed goby, of a white colour, with violet spots on the back; the Japanese scombrus, a beautiful mackerel of these seas, with a blue body and silvery head; the brilliant azurors, whose name alone defies description; some banded spares, with variegated fins of blue and yellow; the woodcocks of the seas, some specimens of which attain a yard in length; Japanese salamanders, spider lampreys, serpents six feet long, with eyes small and lively, and a huge mouth bristling with teeth; with many other species.
I found the little fellow standing in the passage, and being addressed by one of the men in livery, who stood before him, nearly bent double from extreme respectfulness, with his hands hanging in front of him like the fins of a fish.
The animal was only distinguishable, at that distance, by a slight but almost imperceptible motion of its fins and tail.
It was now at the summit of what seemed to be a long white pole, near the top of which were two pendant white masses, like rudimentary arms or fins.