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.
      [1913 Webster]

   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.
      [1913 Webster]

            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
    n 1: a domain in which something is dominant; "the untroubled
         kingdom of reason"; "a land of make-believe"; "the rise of
         the realm of cotton in the south" [syn: kingdom, land,
         realm]
    2: a country with a king as head of state
    3: the domain ruled by a king or queen [syn: kingdom, realm]
    4: a monarchy with a king or queen as head of state
    5: the highest taxonomic group into which organisms are grouped;
       one of five biological categories: Monera or Protoctista or
       Plantae or Fungi or Animalia
    6: a basic group of natural objects

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "kingdom":
   ally, animal kingdom, antonomasia, archduchy, archdukedom, area,
   bailiwick, binomial nomenclature, biological classification,
   biosystematics, biosystematy, biotype, body politic, branch,
   buffer state, captive nation, chain of being, chieftaincy,
   chieftainry, city-state, class, class structure, classification,
   colony, commonweal, commonwealth, country, county, domain,
   dominion, duchy, dukedom, earldom, empery, empire, establishment,
   family, field, free city, genotype, genus, glossology, grand duchy,
   hierarchy, land, mandant, mandate, mandated territory, mandatee,
   mandatory, mineral kingdom, monarchy, nation, nationality,
   natural hierarchy, nomenclature, onomastics, onomatology, order,
   orismology, pecking order, phylum, place-names, place-naming,
   polis, polity, polyonymy, possession, power, power structure,
   principality, principate, protectorate, province,
   puppet government, puppet regime, pyramid, realm, republic,
   satellite, section, seneschalty, series, settlement,
   sovereign nation, sovereignty, species, sphere of influence, state,
   subclass, subfamily, subgenus, subkingdom, suborder, subspecies,
   subtribe, sultanate, superclass, superfamily, superorder,
   superpower, superspecies, systematics, taxonomy, terminology,
   territory, toparchia, toparchy, toponymy, tribe, trinomialism,
   turf, variety, vegetable kingdom