A fin is a surface used to produce lift and thrust or to steer while traveling in water, air, or other fluid 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 ensure the directional stability of an object moving through a fluid such as water or air and may be seen in the use of fletching on arrows and fins at the rear of some missiles, rockets, self-propelled torpedoes, and kinetic energy penetrators.

Fins have also been used on automobiles of the late 1950's and early 1960's, promoted then as adding aerodynamic stability but more now more realistically evaluated as a rather flamboyant style, particularly in American automobiles of this period.

Moving fins may be used to propel an object through lateral thrust (see mechanics).

Examples of fin use:

• Propellers usually have a number of fins that work to translate torquing force to lateral thrust, thus propelling a ship. These are also called blades. In the case of high power application it is important to avoid cavitation, caused by excessive negative pressure, as this can cause noise, a loss of power, and damage to the propeller.
• Scuba divers use fins to better enable themselves to moving through water as human feet provide poor thrust, especially when the diver is carrying equipment that increases drag in the water. Very long fins and monofins are used by freedivers in the search for underwater propulsion that does not require high frequency movements.

Constructions of the same purpose as fins (producing thrust, but working in gaseous media) instead are generally called wings or stabilizers 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 but is (more formally) called a vertical stabilizer.

A fin is also a slang term for a five dollar note; this name was most common during the time of large-sized notes.

Fin is a troll in a Danish legend.

Fin is a historical garden in Kashan, Iran.

Fin is a city in the Hormozgan province of Iran.