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

[Latin Scepticus, disciple of Pyrrho of Elis, from Greek Skeptikos, from skeptesthai, to examine.]

– Source: The American Heritage Dictionaries