The Collaborative International Dictionary of English v.0.48:

Zion \Zi"on\, n. [Heb. ts[imac]y[=o]n, originally, a hill.]
   [1913 Webster]
   1. (Jewish Antiq.) A hill in Jerusalem, which, after the
      capture of that city by the Israelites, became the royal
      residence of David and his successors.
      [1913 Webster]

   2. Hence, the theocracy, or church of God.
      [1913 Webster]

   3. The heavenly Jerusalem; heaven.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Zion
    n 1: originally a stronghold captured by David (the 2nd king of
         the Israelites); above it was built a temple and later the
         name extended to the whole hill; finally it became a
         synonym for the city of Jerusalem; "the inhabitants of
         Jerusalem are personified as `the daughter of Zion'" [syn:
         Zion, Sion]
    2: Jewish republic in southwestern Asia at eastern end of
       Mediterranean; formerly part of Palestine [syn: Israel,
       State of Israel, Yisrael, Zion, Sion]
    3: an imaginary place considered to be perfect or ideal [syn:
       Utopia, Zion, Sion]