The Collaborative International Dictionary of English v.0.48:

Supposition \Sup`po*si"tion\, n. [F. supposition, L. suppositio
   a placing under, a substitution, fr. supponere, suppositium,
   to put under, to substitute. The word has the meaning
   corresponding to suppose. See Sub-, and Position.]
   1. The act of supposing, laying down, imagining, or
      considering as true or existing, what is known not to be
      true, or what is not proved.
      [1913 Webster]

   2. That which is supposed; hypothesis; conjecture; surmise;
      opinion or belief without sufficient evidence.
      [1913 Webster]

            This is only an infallibility upon supposition that
            if a thing be true, it is imposible to be false.
                                                  --Tillotson.
      [1913 Webster]

            He means are in supposition.          --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

supposition
    n 1: a message expressing an opinion based on incomplete
         evidence [syn: guess, conjecture, supposition,
         surmise, surmisal, speculation, hypothesis]
    2: a hypothesis that is taken for granted; "any society is built
       upon certain assumptions" [syn: assumption, supposition,
       supposal]
    3: the cognitive process of supposing [syn: supposition,
       supposal]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "supposition":
   allegory, allusion, apprehension, apriorism, arcane meaning,
   assumption, axiom, belief, coloration, conceit, concept,
   conception, conjecture, connotation, fancy, guesswork, hint,
   hypothesis, idea, image, imago, implication, implied meaning,
   import, impression, inference, innuendo, intellectual object,
   intimation, ironic suggestion, meaning, memory-trace, mental image,
   mental impression, metaphorical sense, notion, nuance, observation,
   occult meaning, opinion, overtone, perception, perhaps, posit,
   postulate, postulation, postulatum, premise, presumption,
   presupposal, presupposition, proposal, proposition, recept,
   reflection, representation, sentiment, set of postulates,
   speculation, subsense, subsidiary sense, suggestion, supposal,
   suppose, supposing, surmise, symbolism, theory, thesis, thought,
   tinge, touch, undercurrent, undermeaning, undertone,
   working hypothesis