The Collaborative International Dictionary of English v.0.48:

Kingdom \King"dom\, n. [AS. cyningd[=o]m. See 2d King, and
   -dom.]
   1. The rank, quality, state, or attributes of a king; royal
      authority; sovereign power; rule; dominion; monarchy.
      [1913 Webster]

            Thy kingdom is an everlasting kingdom. --Ps. cxiv.
                                                  13.
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            When Jehoram was risen up to the kingdom of his
            father, he strengthened himself.      --2 Chron.
                                                  xxi. 4.
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   2. The territory or country subject to a king or queen; the
      dominion of a monarch; the sphere in which one is king or
      has control.
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            Unto the kingdom of perpetual night.  --Shak.
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            You're welcome,
            Most learned reverend sir, into our kingdom. --Shak.
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   3. An extensive scientific division distinguished by leading
      or ruling characteristics; a principal division; a
      department; as, the mineral kingdom. In modern biology,
      the division of life into five kingdoms is widely used for
      classification. "The animal and vegetable kingdoms."
                                                  --Locke.
      [1913 Webster +PJC]

   Animal kingdom. See under Animal.

   Kingdom of God.
      (a) The universe.
      (b) That spiritual realm of which God is the acknowledged
          sovereign.
      (c) The authority or dominion of God.

   Mineral kingdom. See under Mineral.

   United Kingdom. See under United.

   Vegetable kingdom. See under Vegetable.

   Syn: Realm; empire; dominion; monarchy; sovereignty; domain.
        [1913 Webster]

WordNet (r) 3.0 (2006):

Kingdom of God
    n 1: the spiritual domain over which God is sovereign

Easton's 1897 Bible Dictionary:

Kingdom of God
   (Matt. 6:33; Mark 1:14, 15; Luke 4:43) = "kingdom of Christ"
   (Matt. 13:41; 20:21) = "kingdom of Christ and of God" (Eph. 5:5)
   = "kingdom of David" (Mark 11:10) = "the kingdom" (Matt. 8:12;
   13:19) = "kingdom of heaven" (Matt. 3:2; 4:17; 13:41), all
   denote the same thing under different aspects, viz.: (1)
   Christ's mediatorial authority, or his rule on the earth; (2)
   the blessings and advantages of all kinds that flow from this
   rule; (3) the subjects of this kingdom taken collectively, or
   the Church.