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]