fin

¹ American  

[fin] / fɪn /

noun

1. a membranous, winglike or paddlelike organ attached to any of various parts of the body of fishes and certain other aquatic animals, used for propulsion, steering, or balancing.

2. Nautical.

   1. a horizontal, often adjustable, winglike appendage to the underwater portion of a hull, as one for controlling the dive of a submarine or for damping the roll of a surface vessel.

   2. fin keel.

3. Also called vertical stabilizer.  Aeronautics. any of certain small, subsidiary structures on an aircraft, designed to increase directional stability.

4. any of a number of standing ridges on an ordinarily hot object, as a radiator, a cylinder of an internal-combustion engine, etc., intended to maximize heat transfer to the surrounding air by exposing a large surface area.

5. any part, as of a mechanism, resembling a fin.

6. Metallurgy. a ridge of metal squeezed through the opening between two rolls, dies, or halves of a mold in which a piece is being formed under pressure.

7. Automotive. an ornamental structure resembling an aeronautical fin that is attached to the body of an automobile, as on each rear fender tail fin.

8. Slang. the arm or hand.

9. Usually fins. flipper.

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verb (used with object)

finned, finning

1. to cut off the fins from (a fish); carve or cut up, as a chub.

2. to provide or equip with a fin or fins.

verb (used without object)

finned, finning

1. to move the fins; lash the water with the fins, as a whale when dying.

fin ² American  

[fin] / fɪn /

noun

1. Slang. a five-dollar bill.

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fin. ³ American  

abbreviation

1. finance.

2. financial.

3. finish.

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Fin. ⁴ American  

abbreviation

1. Finland.

2. Finnish.

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fin ¹ British  

/ fɪn /

noun

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

2. a part or appendage that resembles a fin

3. 1. US name: vertical stabilizer.  a vertical surface to which the rudder is attached, usually placed at the rear of an aeroplane to give stability about the vertical axis

   2. a tail surface fixed to a rocket or missile to give stability

4. nautical a fixed or adjustable blade projecting under water from the hull of a vessel to give it stability or control

5. a projecting rib to dissipate heat from the surface of an engine cylinder, motor casing, or radiator

6. (often plural) another name for flipper

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verb

1. (tr) to provide with fins

2. (tr) to remove the fins from (a dead fish)

3. (intr) (esp of a whale) to agitate the fins violently in the water

Fin ² British  

abbreviation

1. Finland

2. Finnish

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fin. ³ British  

abbreviation

1. finance

2. financial

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FIN ⁴ British  

abbreviation

1. Finland (international car registration)

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fin ⁵ British  

/ fɪn /

noun

1. slang a five-dollar bill

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fin Scientific  

/ fĭn /

1. One of the winglike or paddlelike parts of a fish, dolphin, or whale that are used for propelling, steering, and balancing in water.

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Other Word Forms

• finless adjective
• finlike adjective

Etymology

Origin of fin¹

First recorded before 1000; Middle English, Old English finn; cognate with Dutch vin, Low German finne; akin to Swedish fena; probably akin to Latin pinna ( def. ); pen ¹ ( def. )

Origin of fin²

First recorded in 1865–70; earlier finnip, finnup, fin(n)if “a five-pound note,” from Yiddish fin(e)f “five,” from Middle High German vumf, vimf; five

Explanation

A fin is the part of a fish's body that protrudes into the water and helps it balance and move in various directions. Sharks use their dorsal fins to stabilize their bodies as they propel through the water. Fish, whales, dolphins, and other aquatic animals have fins on various parts of their bodies. Surfboards often have a rear fin that works in a similar way, helping to keep the board steady and aid in making turns. Ships, cars, and aircraft also have fins that assist with balance. In Old English, it was finn, which probably comes from the Latin pinna, "feather or wing."

Example Sentences

"With its fin immersed into the cell membrane, it rides along the inner surface and swiftly detoxifies lipid peroxides as it goes."

From Science Daily • Dec. 9, 2025

The catalog begins with fin de siècle Europe, when the emergence of mass publishing began to lead to a firmer distinction between subordinate illustrators and visionary fine artists.

From The Wall Street Journal • Nov. 21, 2025

As the local community had previously attempted to rescue the fin whale in 2020, a decision was made to keep part of it so it could eventually be displayed as a memorial.

From BBC • Nov. 18, 2025

Thus, with a flourish, Pynchon types fin to his secret 20th century.

From Los Angeles Times • Sep. 30, 2025

The vuvv speak by grinding a sort of gritty fin against their body, so most of what we said, translated, sounded like sandpaper or, when we were shouting, Velcro.

From "Landscape with Invisible Hand" by M.T. Anderson