The Collaborative International Dictionary of English v.0.48:

Unicity \U*nic"i*ty\, n. [L. unicus single. See Unique.]
   The condition of being united; quality of the unique;
   unification.
   [1913 Webster]

         Not unity, but what the schoolmen call unicity. --De
                                                  Quincey.
   [1913 Webster]

         The unicity we strive not to express, for that is
         impossible, but to designate by the nearest analogy.
                                                  --Coleridge.
   [1913 Webster]