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]