A pork pie hat, or trilby, is a felt hat, dating from the middle 18th century, much the same as a fedora, but with a flattened top. It gets its name from its resemblance to a pork pie. The brim on a pork pie hat is generally on the smaller side, and is worn up, though it can be worn down in the front. The hats come in straw varieties as well.
Pork pie hats are often associated with jazz culture, though more recently they have had strong associations with ska. In Britain they were opularized in the 1960s Rude Boy movement in Britain; then later adopted by the skinheads. The Pork-Pie was a staple of the British Man-About-Town for many years, before its association with any particular youth subculture.