The Collaborative International Dictionary of English v.0.48:

Coherence \Co*her"ence\, Coherency \Co*her"en*cy\, n. [L.
   cohaerentia: cf. F. coh['e]rence.]
   1. A sticking or cleaving together; union of parts of the
      same body; cohesion.
      [1913 Webster]

   2. Connection or dependence, proceeding from the
      subordination of the parts of a thing to one principle or
      purpose, as in the parts of a discourse, or of a system of
      philosophy; a logical and orderly and consistent relation
      of parts; consecutiveness.
      [1913 Webster]

            Coherence of discourse, and a direct tendency of all
            the parts of it to the argument in hand, are most
            eminently to be found in him.         --Locke.
      [1913 Webster]

   3. the state of cohering.

   Syn: cohesion, cohesiveness.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

coherency
    n 1: the state of cohering or sticking together [syn:
         coherence, coherency, cohesion, cohesiveness] [ant:
         incoherence, incoherency]
    2: logical and orderly and consistent relation of parts [syn:
       coherence, coherency]