A roundel in heraldry is any circular shape; in military use it is an emblem of nationality employed on military aircraft and air force flags, generally round and consisting of concentric rings of different colors.
In French blazon, a roundel of either metal (or or argent) is a besant, and a roundel of any colour (dark tincture) is a torteau, with the tincture specified.
One special example of a named roundel is the fountain. This is depicted as a roundel barry wavy argent and azure. That is, it is not filled in a single tincture, but with alternating horizontal wavy bands of blue and silver (or white). Because the fountain consists equally of parts in a light and a dark tincture, its use is not limited by the rule of tincture as are the other roundels.
In their earliest uses, roundels were often strewn (semy) upon the field of a coat of arms, a design with as many names as there are tinctures. For example, a field semy of roundels argent could be called platy; a field semy of roundels sable could be called pellety. The precise number and placement of the roundels in such cases were usually left to the discretion of the artist.
Because of their long use and simple outline, roundels are accorded status as a subordinary charge by most heraldic writers.
The term roundel also describes a circular shield used for heraldic display (as opposed to other forms such as the more common escutcheon or lozenge). An example of arms borne on a roundel is the Coat of Arms of Nunavut.
The first use of a roundel on military aircraft was during the First World War by the French Air Service. The chosen design was the French national cockade, which consisted of a blue-white-red emblem mirroring the colours of the Flag of France. Similar national cockades were designed and adopted for use as aircraft roundels by their allies. The British Royal Flying Corps (RFC) abandoned their original painted Union Flags because they looked too much like the German cross and the British soldiers in the trenches had shot at them mistaking them for German aircraft. Instead, British aircraft roundels were eventually designed to use the French colours in reverse, red-white-blue from center to rim, after a short period of using a red-rimmed white circle that closely resembled the roundel, used to this day by Denmark. This red-white-blue from center to rim roundel design was subsequently adopted by British Imperial air forces with a maple leaf (RCAF), kangaroo (RAAF), and kiwi (RNZAF). During WWI, roundels based on national cockade colours were subsequently adopted by the air forces of other countries, including the U.S. Army Air Service.
The military aviation insignia of both the United States and Russia have had interesting "crossovers" early in the 20th century. The initial US Army Air Service insignia, used during the Pancho Villa punitive expedition just before World War I, used on the vertical tail and wings (as seen on early Curtiss biplane two-seat observation aircraft), was essentially the red star of the slightly later Soviet Union, without a red or white outline border. The Imperial Russian tricolor roundel was also adopted by the US Army Air Service, in more evenly spaced proportions than the Tsarist marking, in World War I air combat in Europe, because the then-recently adopted "star-in-circle" insignia could possibly be mistaken for a German or Austro-Hungarian "Eisernes Kreuz" (iron cross) insignia at some distance. The US roundel added a pair of white bars during World War II, becoming the famous "Stars and Bars" insignia, a red bar bisecting the white was added in 1947 when the United States Air Force became a separate service.
During the Second World War, the red inner circle of RAF-based roundels on planes based in the Asia-Pacific was painted white or light blue, so they would not be confused for the Hinomaru red circle on Japanese planes, still used by the self-defense forces of Japan to this day.
In recent decades, "low-visibility" roundels have been used, especially on active combat types. Low-visibility roundels usually have subdued, low-contrast colors or stenciled outlines.