tiara

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

• noun An ornamental, often jeweled, crownlike semicircle worn on the head by women on formal occasions.
• noun The triple crown historically worn by the pope, especially at his coronation ceremony, and still used heraldically as a symbol of the papacy.

from The Century Dictionary.

• noun An ornament or article of dress with which the ancient Persians covered the head: a kind of turban.
• noun A cylindrical diadem pointed at the top, tipped with the mound and cross of sovereignty, and surrounded with three crowns, which the Pope wears as a symbol of his threefold sovereignty.
• noun Figuratively, the papal dignity.
• noun A coronet or frontal; an ornament for the head: used loosely for any such ornament considered unusually rich: as, a tiara of brilliants.
• noun In heraldry, a bearing representing a tall cap-like or pointed dome surrounded by three crowns, one above the other, and having at the point an orb and cross: it is supposed to represent the crown of the Pope.
• noun In conchology: A miter-shell.
• noun [capitalized] A genus of miter-shells.

from the GNU version of the Collaborative International Dictionary of English.

• noun A form of headdress worn by the ancient Persians. According to Xenophon, the royal tiara was encircled with a diadem, and was high and erect, while those of the people were flexible, or had rims turned over.
• noun The pope's triple crown. It was at first a round, high cap, but was afterward encompassed with a crown, subsequently with a second, and finally with a third. Fig.: The papal dignity.

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

• noun The papal crown.
• noun An ornamental coronet.

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

• noun a jeweled headdress worn by women on formal occasions