The Collaborative International Dictionary of English v.0.48:

Synchronism \Syn"chro*nism\, n. [Gr. ?, fr. ? to be contemporary
   with, from ? synchronous. See Synchronous.]
   1. The concurrence of events in time; simultaneousness.
      [1913 Webster]

   2. The tabular arrangement of historical events and
      personages, according to their dates.
      [1913 Webster]

   3. (Paint.) A representation, in the same picture, of two or
      events which occured at different times.
      [1913 Webster]

WordNet (r) 3.0 (2006):

synchronism
    n 1: the relation that exists when things occur at the same
         time; "the drug produces an increased synchrony of the
         brain waves" [syn: synchronism, synchrony,
         synchronicity, synchroneity, synchronization,
         synchronisation, synchronizing] [ant: asynchronism,
         asynchrony, desynchronisation, desynchronization,
         desynchronizing]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "synchronism":
   accompaniment, accord, accordance, affinity, agreement, alliance,
   assent, association, attune, attunement, cahoots, chime, chiming,
   chorus, co-occurrence, co-working, coaction, coetaneity,
   coetaneousness, coevalneity, coevalness, coexistence, coherence,
   coincidence, collaboration, collectivity, collusion, combination,
   combined effort, compatibility, concentus, concert,
   concerted action, concomitance, concomitancy, concord, concordance,
   concourse, concurrence, confluence, conformance, conformation,
   conformity, congeniality, congruence, congruency, congruity,
   conjunction, consilience, consistency, consonance, consonancy,
   consort, conspiracy, contemporaneity, contemporaneousness,
   cooperation, correspondence, diapason, equivalence, euphony,
   harmonics, harmony, heavy harmony, homophony, intersection,
   isochronism, junction, monochord, monody, oneness, overlap,
   parallelism, parasitism, peace, rapport, saprophytism,
   self-consistency, simultaneity, symbiosis, symmetry, symphony,
   sync, synchronization, synergy, tally, three-part harmony, timing,
   togetherness, tune, uniformity, union, unison, unisonance,
   united action, withness