The Collaborative International Dictionary of English v.0.48:

Dominion \Do*min"ion\, n. [LL. dominio, equiv. to L. dominium.
   See Domain, Dungeon.]
   1. Sovereign or supreme authority; the power of governing and
      controlling; independent right of possession, use, and
      control; sovereignty; supremacy.
      [1913 Webster]

            I praised and honored him that liveth forever, whose
            dominion is an everlasting dominion.  --Dan. iv. 34.
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            To choose between dominion or slavery. --Jowett
                                                  (Thucyd. ).
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   2. Superior prominence; predominance; ascendency.
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            Objects placed foremost ought . . . have dominion
            over things confused and transient.   --Dryden.
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   3. That which is governed; territory over which authority is
      exercised; the tract, district, or county, considered as
      subject; as, the dominions of a king. Also used
      figuratively; as, the dominion of the passions.
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   4. pl. A supposed high order of angels; dominations. See
      Domination, 3. --Milton.
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            By him were all things created . . . whether they be
            thrones, or dominions, or principalities, or powers.
                                                  --Col. i. 16.

   Syn: Sovereignty; control; rule; authority; jurisdiction;
        government; territory; district; region.
        [1913 Webster]

WordNet (r) 3.0 (2006):

dominion
    n 1: dominance or power through legal authority; "France held
         undisputed dominion over vast areas of Africa"; "the rule
         of Caesar" [syn: dominion, rule]
    2: a region marked off for administrative or other purposes
       [syn: district, territory, territorial dominion,
       dominion]
    3: one of the self-governing nations in the British Commonwealth