The Collaborative International Dictionary of English v.0.48:

Tiara \Ti*a"ra\, n. [L., from Gr. ?, ?; of Persian origin.]
   1. 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.
      [1913 Webster]

   2. 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tiara
    n 1: a jeweled headdress worn by women on formal occasions

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "tiara":
   anklet, armlet, bangle, beads, bijou, bracelet, breastpin, brooch,
   cap of dignity, cap of maintenance, chain, chaplet, charm,
   chatelaine, circle, coronet, crook, crosier, crown, diadem,
   earring, ermine, fob, gem, great seal, jewel, keys, locket, miter,
   necklace, nose ring, orb, pallium, pastoral staff, pin,
   precious stone, privy seal, purple, purple pall, red hat, regalia,
   rhinestone, ring, robe of state, rod, rod of empire, royal crown,
   scepter, seal, signet, stickpin, stone, torque, triple crown,
   triple plume, uraeus, wampum, wristband, wristlet