skeptic

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One who instinctively or habitually doubts, questions, or disagrees with assertions or generally accepted conclusions.
2. n. One inclined to skepticism in religious matters.
3. n. Philosophy An adherent of a school of skepticism.
4. n. Philosophy A member of an ancient Greek school of skepticism, especially that of Pyrrho of Elis (360?-272? B.C.).

Century Dictionary and Cyclopedia

1. Same as skeptical.
2. n. 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.
3. n. One who doubts or disbelieves the fundamental principles of the Christian religion.
4. n. An adherent of a philosophical school in ancient Greece. The first group of this school consisted of Pyrrho and his immediate followers (see Pyrrhonic); the second group formed the so-called Middle Academy, less radical than Pyrrho; and the third group (Ænesidemus in the first century, Sextus, etc.) returned in part to the doctrines of Pyrrho. Ueberweg.
5. n. One who doubts concerning the truth of any particular proposition; one who has a tendency to question the virtue and integrity of most persons.
6. n. Synonyms Unbeliever, Free-thinker, etc. See infidel.

Wiktionary

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

GNU Webster's 1913

1. n. 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.
2. n. (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.
3. n. (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.
4. adj. Of or pertaining to a sceptic or skepticism; characterized by skepticism; hesitating to admit the certainly of doctrines or principles; doubting of everything.
5. adj. (Theol.) Doubting or denying the truth of revelation, or the sacred Scriptures.

WordNet 3.0

1. n. someone who habitually doubts accepted beliefs