The Collaborative International Dictionary of English v.0.48:

Agnosticism \Ag*nos"ti*cism\, n.
   That doctrine which, professing ignorance, neither asserts
   nor denies. Specifically: (Theol.) The doctrine that the
   existence of a personal Deity, an unseen world, etc., can be
   neither proved nor disproved, because of the necessary limits
   of the human mind (as sometimes charged upon Hamilton and
   Mansel), or because of the insufficiency of the evidence
   furnished by physical and physical data, to warrant a
   positive conclusion (as taught by the school of Herbert
   Spencer); -- opposed alike dogmatic skepticism and to
   dogmatic theism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

agnosticism
    n 1: a religious orientation of doubt; a denial of ultimate
         knowledge of the existence of God; "agnosticism holds that
         you can neither prove nor disprove God's existence"
    2: the disbelief in any claims of ultimate knowledge [syn:
       agnosticism, skepticism, scepticism]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "agnosticism":
   Humism, Pyrrhonism, atheism, blankmindedness, callowness, denial,
   disbelief, discredit, doubt, empty-headedness, greenhornism,
   greenness, heresy, hiatus of learning, ignorance, ignorantism,
   ignorantness, inability to believe, inanity, incredulity,
   inexperience, infidelity, innocence, know-nothingism,
   knowledge-gap, lack of information, minimifidianism, misbelief,
   nescience, nonbelief, nullifidianism, obscurantism, rawness,
   rejection, scoffing, simpleness, simplicity, skepticism,
   tabula rasa, unacquaintance, unbelief, unbelievingness,
   unfamiliarity, unintelligence, unknowing, unknowingness,
   unripeness, vacuity, vacuousness