The Collaborative International Dictionary of English v.0.48:

Skeptic \Skep"tic\, n. [Gr. skeptiko`s thoughtful, reflective,
   fr. ske`ptesqai to look carefully or about, to view,
   consider: cf. L. scepticus, F. sceptique. See Scope.]
   [Written also sceptic.]
   1. 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.
      [1913 Webster]

   2. (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.
      [1913 Webster]

            All this criticism [of Hume] proceeds upon the
            erroneous hypothesis that he was a dogmatist. He was
            a skeptic; that is, he accepted the principles
            asserted by the prevailing dogmatism: and only
            showed that such and such conclusions were, on these
            principles, inevitable.               --Sir W.
                                                  Hamilton.
      [1913 Webster]

   3. (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.
      [1913 Webster]

            Suffer not your faith to be shaken by the
            sophistries of skeptics.              --S. Clarke.
      [1913 Webster]

   Note: This word and its derivatives are often written with c
         instead of k in the first syllable, -- sceptic,
         sceptical, scepticism, etc. Dr. Johnson, struck with
         the extraordinary irregularity of giving c its hard
         sound before e, altered the spelling, and his example
         has been followed by most of the lexicographers who
         have succeeded him; yet the prevalent practice among
         English writers and printers is in favor of the other
         mode. In the United States this practice is reversed, a
         large and increasing majority of educated persons
         preferring the orthography which is most in accordance
         with etymology and analogy.
         [1913 Webster]

   Syn: Infidel; unbeliever; doubter. -- See Infidel.
        [1913 Webster] Skeptic

The Collaborative International Dictionary of English v.0.48:

Skeptic \Skep"tic\, Skeptical \Skep"tic*al\, a. [Written also
   sceptic, sceptical.]
   1. Of or pertaining to a sceptic or skepticism; characterized
      by skepticism; hesitating to admit the certainly of
      doctrines or principles; doubting of everything.
      [1913 Webster]

   2. (Theol.) Doubting or denying the truth of revelation, or
      the sacred Scriptures.
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            The skeptical system subverts the whole foundation
            of morals.                            --R. Hall.
      [1913 Webster] -- Skep"tac*al*ly, adv. --
      Skep"tic*al*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

skeptic
    n 1: someone who habitually doubts accepted beliefs [syn:
         skeptic, sceptic, doubter]