a soft, closefitting cap without a visor, adopted by the French Revolutionists as a symbol of liberty
See liberty cap in American Heritage Dictionary 4
A brimless, limp, conical cap fitting snugly around the head and given to a slave in ancient Rome upon manumission. It was used as a symbol of liberty by the French revolutionaries and was also worn in the United States before 1800. Also called Phrygian cap.