The Collaborative International Dictionary of English v.0.48:

Dissenter \Dis*sent"er\, n.
   1. One who dissents; one who differs in opinion, or declares
      his disagreement.
      [1913 Webster]

   2. (Eccl.) One who separates from the service and worship of
      an established church; especially, one who disputes the
      authority or tenets of the Church of England; a
      nonconformist.
      [1913 Webster]

            Dissenters from the establishment of their several
            countries.                            --Burke.
      [1913 Webster]

            Robert Brown is said to have the first formal
            dissenter.                            --Shipley.
      [1913 Webster]

   Note: "The word is commonly applied only to Protestants. The
         Roman Catholics are generally referred to as a distinct
         class." --Brande & C.
         [1913 Webster]

WordNet (r) 3.0 (2006):

dissenter
    n 1: a person who dissents from some established policy [syn:
         dissenter, dissident, protester, objector,
         contestant]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "dissenter":
   Bohemian, Evangelical, Protestant, Reformationist,
   Reformed believer, Zwinglian, apostate, beatnik, demurrer, deviant,
   dissentient, dissident, dropout, flower child, freak, heretic,
   hippie, maverick, misbeliever, misfit, non-Catholic, nonconformist,
   nonjuror, objector, opinionist, opposition voice, original,
   protestant, protester, recusant, schismatic, sectarian, sectary,
   separatist, swinger, ugly duckling, unconformist, yippie