The Collaborative International Dictionary of English v.0.48:

Disunion \Dis*un"ion\, n. [Pref. dis- + union: cf. F.
   d['e]sunion.]
   1. The termination of union; separation; disjunction; as, the
      disunion of the body and the soul.
      [1913 Webster]

   2. A breach of concord and its effect; alienation.
      [1913 Webster]

            Such a disunion between the two houses as might much
            clou? the happiness of this kingdom.  --Clarendon.
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   3. The termination or disruption of the union of the States
      forming the United States.
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            I have not accustomed myself to hang over the
            precipice of disunion.                --D. Webster.
      [1913 Webster]

WordNet (r) 3.0 (2006):

disunion
    n 1: the termination or destruction of union [ant:
         conjugation, jointure, unification, union,
         uniting]