skeptic

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

• noun One who instinctively or habitually doubts, questions, or disagrees with assertions or generally accepted conclusions.
• noun One inclined to skepticism in religious matters.
• noun An adherent of a school of skepticism.
• noun A member of an ancient Greek school of skepticism, especially that of Pyrrho of Elis (360?–272? BC).

from The Century Dictionary.

• Same as skeptical.
• noun One who suspends his judgment, and holds that the known facts do not warrant a conclusion concerning a given fundamental question; a thinker distinguished for the length to which he carries his doubts; also, one who holds that the real truth of things cannot be known in any case; one who will not affirm or deny anything in regard to reality as opposed to appearance.
• noun One who doubts or disbelieves the fundamental principles of the Christian religion.
• noun An adherent of a philosophical school in ancient Greece.
• noun One who doubts concerning the truth of any particular proposition; one who has a tendency to question the virtue and integrity of most persons.
• noun Synonyms Unbeliever, Free-thinker, etc. See infidel.

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

• noun One who is yet undecided as to what is true; one who is looking or inquiring for what is true; an inquirer after facts or reasons.
• noun (Metaph.) A doubter as to whether any fact or truth can be certainly known; a universal doubter; a Pyrrhonist; hence, in modern usage, occasionally, a person who questions whether any truth or fact can be established on philosophical grounds; sometimes, a critical inquirer, in opposition to a dogmatist.
• noun (Theol.) A person who doubts the existence and perfections of God, or the truth of revelation; one who disbelieves the divine origin of the Christian religion.
• adjective Of or pertaining to a sceptic or skepticism; characterized by skepticism; hesitating to admit the certainly of doctrines or principles; doubting of everything.
• adjective (Theol.) Doubting or denying the truth of revelation, or the sacred Scriptures.

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

• noun Someone who habitually doubts beliefs and claims presented as accepted by others, requiring strong evidence before accepting any belief or claim.
• noun Someone undecided as to what is true.
• noun A type of agnostic

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

• noun someone who habitually doubts accepted beliefs