The Collaborative International Dictionary of English v.0.48:

Presupposition \Pre*sup`po*si"tion\, n. [Pref. pre- +
   supposition: cf. F. pr['e]supposition.]
   1. The act of presupposing; an antecedent implication;
      presumption.
      [1913 Webster]

   2. That which is presupposed; a previous supposition or
      surmise.
      [1913 Webster]

WordNet (r) 3.0 (2006):

presupposition
    n 1: the act of presupposing; a supposition made prior to having
         knowledge (as for the purpose of argument)

Moby Thesaurus II by Grady Ward, 1.0:

140 Moby Thesaurus words for "presupposition":
   a priori principle, act, affect, affirmation, allegory, allusion,
   apriorism, arcane meaning, assertion, assumed position, assumption,
   avant-propos, axiom, basis, beguile, belief, bluff, breakthrough,
   categorical proposition, coloration, conjecture, connotation,
   conviction, counterfeit, data, deceive, deduction, delude,
   exordium, fake, feign, first principles, forejudgment, foreword,
   foundation, front matter, frontispiece, ground, guess, guesswork,
   hint, hypothesis, hypothesis ad hoc, implication, implied meaning,
   import, inference, innovation, innuendo, intimation, introduction,
   ironic suggestion, judgment, leap, lemma, major premise,
   make believe, meaning, metaphorical sense, minor premise, mislead,
   nuance, occult meaning, opinion, overtone, overture, parti pris,
   philosopheme, philosophical proposition, posit, position,
   postulate, postulation, postulatum, preamble, preapprehension,
   preconception, preconclusion, preconsideration, predecision,
   predetermination, predilection, predisposition, preface, prefix,
   prefixture, prejudgment, prejudication, prejudice, preliminary,
   prelude, premature judgment, premise, prenotion, prepossession,
   presume, presumption, presupposal, presurmise, pretend, proem,
   profess, prolegomena, prolegomenon, prolepsis, prologue,
   proposition, propositional function, protasis, purport, put on,
   set of postulates, sham, simulate, statement, subsense,
   subsidiary sense, suggestion, sumption, supposal, suppose,
   supposing, supposition, surmise, symbolism, theorem, thesis, think,
   tinge, touch, truth table, truth-function, truth-value,
   undercurrent, undermeaning, undertone, verse, view, voluntary,
   working hypothesis