liberty cap

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from The Century Dictionary.

• noun A cap of the form known as the Phrygian, used as a symbol of political or personal liberty.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a brimless felt cap, such as the Phrygian cap or pileus, emblematic of a slave's manumission in the Ancient World.
• noun a magic mushroom, Psilocybe semilanceata

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun close-fitting conical cap worn as a symbol of liberty during the French Revolution and in the U.S. before 1800