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