The bicorne or bicorn is an archaic form of hat widely adopted in the 1790s as an item of uniform by Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

an and American military

Army

An army An army An army (from Latin arma "arms, weapons" via Old French armée, "armed" (feminine), in the broadest sense, is the land-based military of a nation or state. It may also include other branches of the military such as the air force via means of aviation corps...

 and naval

Navy

A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...

 officers

Officer (armed forces)

An officer is a member of an armed force or uniformed service who holds a position of authority. Commissioned officers derive authority directly from a sovereign power and, as such, hold a commission charging them with the duties and responsibilities of a specific office or position...

. It is now most readily associated with Napoléon Bonaparte but in practice most generals and staff officers of the Napoleonic period wore bicornes, and it survived as a widely worn full-dress headdress until at least 1914.
Descended from the tricorne

Tricorne

The tricorne or tricorn is a style of hat that was popular during the 18th century, falling out of style by 1800. At the peak of its popularity, the tricorne was worn as civilian dress and as part of military and naval uniforms...

, the black-coloured bicorne originally had a rather broad brim, with the front and the rear halves turned up and pinned together (in English the shorter front brim was called 'the cock' - hence 'cocked hat' - and the longer rear brim was termed 'the fan'), forming a semi-circular fan shape; there was usually a cockade

Cockade

A cockade is a knot of ribbons, or other circular- or oval-shaped symbol of distinctive colors which is usually worn on a hat.-Eighteenth century:...

 in the national colours at the front.