A derby is a type of hat that's round, felt, stiff, and has a narrow brim. Wear one with a little mustache and a cane, and you’ll look just like Charlie Chaplin.
A derby, as the distinctive round hat with a little brim, was made in the U.S. in the 1850s, but it’s probably named after the Derby horse race in England, where men wore this kind of hat. The race was named after the 12th Earl of Derby. A derby can also be called a bowler, bowler hat, derby hat, or plug hat. Derbies are kind of old-fashioned, so you probably won't see anyone wearing one these days.