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A fin is a surface used to produce thrust by travelling in water or another liquid media. The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man made devices.

The foremost use of fins are to propel an object through lateral thrust (see mechanics).

Fin construction is complicated and very tightly associated with the science of hydrodynamics. Fin construction is much centered around avoiding cavitation.

Examples of fin use:

Constructions of the same purpose as fins, but working in gaseous media instead are generally called wings with aerodynamics as the governing science. The exception to this is the vertical surface of an aircraft to which the rudder is attached - this is still usually called a fin.
A fin is also a slang term for a five dollar note.