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]