The Collaborative International Dictionary of English v.0.48:

Skepticism \Skep"ti*cism\, n. [Cf. F. scepticisme.] [Written
   also scepticism.]
   1. An undecided, inquiring state of mind; doubt; uncertainty.
      [1913 Webster]

            That momentary amazement, and irresolution, and
            confusion, which is the result of skepticism.
                                                  --Hune.
      [1913 Webster]

   2. (Metaph.) The doctrine that no fact or principle can be
      certainly known; the tenet that all knowledge is
      uncertain; Pyrrohonism; universal doubt; the position that
      no fact or truth, however worthy of confidence, can be
      established on philosophical grounds; critical
      investigation or inquiry, as opposed to the positive
      assumption or assertion of certain principles.
      [1913 Webster]

   3. (Theol.) A doubting of the truth of revelation, or a
      denial of the divine origin of the Christian religion, or
      of the being, perfections, or truth of God.
      [1913 Webster]

            Let no . . . secret skepticism lead any one to doubt
            whether this blessed prospect will be realized. --S.
                                                  Miller.
      [1913 Webster]

WordNet (r) 3.0 (2006):

skepticism
    n 1: doubt about the truth of something [syn: incredulity,
         disbelief, skepticism, mental rejection]
    2: the disbelief in any claims of ultimate knowledge [syn:
       agnosticism, skepticism, scepticism]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "skepticism":
   Humism, Pyrrhonism, agnosticism, apprehension, caution,
   cautiousness, concern, diffidence, disinclination to believe,
   distrust, distrustfulness, doubt, doubtfulness, dubiety,
   dubiousness, guardedness, half-belief, incertitude,
   inconvincibility, incredulity, leeriness, misdoubt, misgiving,
   mistrust, mistrustfulness, qualm, qualmishness, question,
   resistiveness to belief, scoffing, scruple, scrupulousness,
   self-doubt, shadow of doubt, skepticalness, suspicion,
   suspiciousness, total skepticism, tough-mindedness, uncertainty,
   unconvincibility, uncredulousness, unpersuadability,
   unpersuasibility, wariness, wonder