dissident

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Disagreeing, as in opinion or belief.
• noun One who disagrees; a dissenter.

from The Century Dictionary.

• Different; at variance.
• Dissenting; not conforming; specifically, dissenting from an established church.
• noun One who differs or dissents from others in regard to anything; especially, an opponent of or dissenter from a prevailing opinion, method, etc.
• noun Specifically— A dissenter; one who separates from an established religion.
• noun Especially— Under the old elective monarchy of Poland, when the established church was Roman Catholic, a Lutheran, Calvinist, Arminian, or adherent of the Greek Church, who was allowed the free exercise of his faith.

from the GNU version of the Collaborative International Dictionary of English.

• adjective No agreeing; dissenting; discordant; different.
• noun (Eccl.) One who disagrees or dissents; one who separates from the established religion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective In a manner that disagrees; dissenting; discordant; different.
• noun A person who formally opposes the current political structure, opposes the political group in power, opposes the policies of the political group in power, or opposes current laws.
• noun ecclesiastical One who disagrees or dissents; one who separates from the established religion.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective disagreeing, especially with a majority
• noun a person who dissents from some established policy
• adjective characterized by departure from accepted beliefs or standards