Source : Webster's Revised Unabridged Dictionary (1913)

Congregationalist \Con`gre*ga"tion*al*ist\, n.
   One who belongs to a Congregational church or society; one
   who holds to Congregationalism.

Source : WordNet®

Congregationalist
     adj : of or pertaining to or characteristic of a Congregational
           church [syn: {Congregational}]
     n : a member of the Congregational Church