"Derby hats" refers to wide-brimmed and usually formal hats that are commonly worn at horse racing events called derbies or races, as a fashionable and traditional accessory. Full definition
Dress accordingly; a prize will be awarded for «best derby hat». (thebostondaybook.com)
It will be a perfect derby hat and it is great for just hanging out in the sun. (explorelovelive.com)
In honor of the opening of the Del Mar Racetrack today and the annual hat contest, I have decided to do a little DIY derby hat post for you all! (lauralily.com)