The tricorne, or tricorn, is a style of hat that was popular during the 18th century, falling out of style by the early 1800s. It wasn't called a "tricorne" until the mid-19th century. During the 18th century, hats of this general style were referred to as "cocked hats". At the peak of its popularity, the tricorne varied greatly in style and size and was worn not only by the aristocracy but also as common civilian dress, and as part of military and naval uniforms.

Source: Wikipedia