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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 shipA ship like a boat, is a vehicle designed for passage or transportation by water. A ship usually has sufficient size to carry its own boats, such as lifeboats, dinghies, or runabouts. A rule of thumb saying (though it doesn't always apply) goes: "a boat c. These are also called blades. In the case of high power application it is important to avoid cavitationCavitation is the phenomenon where small cavities of partial vacuum form in fluid then rapidly collapse, producing a sharp sound. Pump Cavitation occurs in pumps, as well as around propellers, or at restrictions in a flowing liquid. Cavitation means that, 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 humanHuman beings are defined variously in biological, spiritual, and cultural terms, or in combinations thereof. Biologically, they are classified as Homo sapiens ( Latin for knowing man , a primate species of mammal with a highly developed brain. In spiritua feet provide poor thrust, especially when the diver is carrying equipment that increases dragFor a solid object moving through a fluid, drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the external fluid flow. It therefore acts to oppose the motion of the object, and in a powered vehicle it is overcome by thrust. in the water. Very long fins and monofinA monofin is a type of swimfin typically used in free-diving. It consists of a single surface attached to footpockets for both the free-diver's feet. By slowly oscillating the surface of the monofin when submerged the freediver can generate large amountss are used by freedivers in the search for underwater propulsion that does not require high frequencyFrequency is the measurement of the number of times that a repeated event occurs per unit time. To calculate the frequency, one fixes a time interval, counts the number of occurrences of the event within that interval, and then divides this count by the l 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.