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.
a.
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.
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. Compare flash(def. 11).
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).
fin (fĭn) Pronunciation Key
One of the winglike or paddlelike parts of a fish, dolphin, or whale that are used for propelling, steering, and balancing in water.
[fin] 
noun, verb, finned, fin⋅ning.