Source : Webster's Revised Unabridged Dictionary (1913)

Congregationalism \Con`gre*ga"tion*al*ism\, n.
   1. That system of church organization which vests all
      ecclesiastical power in the assembled brotherhood of each
      local church.

   2. The faith and polity of the Congregational churches, taken
      collectively.

   Note: In this sense (which is its usual signification)
         Congregationalism is the system of faith and practice
         common to a large body of evangelical Trinitarian
         churches, which recognize the local brotherhood of each
         church as independent of all dictation in
         ecclesiastical matters, but are united in fellowship
         and joint action, as in councils for mutual advice, and
         in consociations, conferences, missionary
         organizations, etc., and to whose membership the
         designation ``Congregationalists'' is generally
         restricted; but Unitarian and other churches are
         Congregational in their polity.

Source : WordNet®

Congregationalism
     n : system of beliefs and church government of a Protestant
         denomination in which each member church is
         self-governing