The Collaborative International Dictionary of English v.0.48:

Coincidence \Co*in"ci*dence\ (k[-o]*[i^]n"s[i^]*dens), n. [Cf.
   F. co["i]ncidence.]
   1. The condition of occupying the same place in space; as,
      the coincidence of circles, surfaces, etc. --Bentley.
      [1913 Webster]

   2. The condition or fact of happening at the same time; as,
      the coincidence of the deaths of John Adams and Thomas
      Jefferson.
      [1913 Webster]

   3. Exact correspondence in nature, character, result,
      circumstances, etc.; concurrence; agreement.
      [1913 Webster]

            The very concurrence and coincidence of so many
            evidences . . . carries a great weight. --Sir M.
                                                  Hale.
      [1913 Webster]

            Those who discourse . . . of the nature of truth . .
            . affirm a perfect coincidence between truth and
            goodness.                             --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

coincidence
    n 1: an event that might have been arranged although it was
         really accidental [syn: coincidence, happenstance]
    2: the quality of occupying the same position or area in space;
       "he waited for the coincidence of the target and the cross
       hairs"
    3: the temporal property of two things happening at the same
       time; "the interval determining the coincidence gate is
       adjustable" [syn: concurrence, coincidence,
       conjunction, co-occurrence]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "coincidence":
   accident, accompaniment, accord, accordance, affinity, agreement,
   alliance, assent, association, cahoots, chance, chorus,
   co-occurrence, co-working, coaction, coequality, coetaneity,
   coetaneousness, coevality, coevalneity, coevalness, coexistence,
   coextension, coherence, collaboration, collectivity, collusion,
   combination, combined effort, compatibility, concert,
   concerted action, concomitance, concomitancy, concord, concordance,
   concourse, concurrence, confluence, conformance, conformation,
   conformity, congeniality, congruence, congruency, congruity,
   conjunction, consilience, consistency, consonance, consort,
   conspiracy, contemporaneity, contemporaneousness, cooperation,
   correspondence, equality, equivalence, fluke, fortuitousness,
   fortuity, happenstance, harmony, homogeneity, homoousia, identity,
   indistinguishability, intersection, isochronism, junction, luck,
   matching, no difference, oneness, overlap, parallelism, parasitism,
   peace, rapport, sameness, saprophytism, self-consistency,
   self-identity, selfhood, selfness, selfsameness, simultaneity,
   symbiosis, symmetry, sync, synchronism, synchronization, synergy,
   synonymity, synonymousness, synonymy, tally, timing, togetherness,
   uniformity, union, unison, unisonance, united action, unity,
   withness