The Collaborative International Dictionary of English v.0.48:

Hypothesis \Hy*poth"e*sis\, n.; pl. Hypotheses. [NL., fr. Gr.
   ? foundation, supposition, fr. ? to place under, ? under + ?
   to put. See Hypo-, Thesis.]
   1. A supposition; a proposition or principle which is
      supposed or taken for granted, in order to draw a
      conclusion or inference for proof of the point in
      question; something not proved, but assumed for the
      purpose of argument, or to account for a fact or an
      occurrence; as, the hypothesis that head winds detain an
      overdue steamer.
      [1913 Webster]

            An hypothesis being a mere supposition, there are no
            other limits to hypotheses than those of the human
            imagination.                          --J. S. Mill.
      [1913 Webster]

   2. (Natural Science) A tentative theory or supposition
      provisionally adopted to explain certain facts, and to
      guide in the investigation of others; hence, frequently
      called a working hypothesis.

   Syn: Supposition; assumption. See Theory.
        [1913 Webster]

   Nebular hypothesis. See under Nebular. Hypothetic

WordNet (r) 3.0 (2006):

hypothesis
    n 1: a proposal intended to explain certain facts or
         observations
    2: a tentative insight into the natural world; a concept that is
       not yet verified but that if true would explain certain facts
       or phenomena; "a scientific hypothesis that survives
       experimental testing becomes a scientific theory"; "he
       proposed a fresh theory of alkalis that later was accepted in
       chemical practices" [syn: hypothesis, possibility,
       theory]
    3: a message expressing an opinion based on incomplete evidence
       [syn: guess, conjecture, supposition, surmise,
       surmisal, speculation, hypothesis]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "hypothesis":
   a priori principle, affirmation, apriorism, assertion,
   assumed position, assumption, axiom, basis,
   categorical proposition, conjecture, data, first principles,
   foundation, ground, guesswork, hypothesis ad hoc, inference, lemma,
   major premise, minor premise, philosopheme,
   philosophical proposition, position, postulate, postulation,
   postulatum, premise, premiss, presumption, presupposal,
   presupposition, proposition, propositional function,
   set of postulates, speculation, statement, sumption, supposal,
   supposing, supposition, surmise, theorem, theory, thesis,
   truth table, truth-function, truth-value, working hypothesis