Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

   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}.

   {Nebular hypothesis}. See under {Nebular}.

Source : WordNet®

hypothesis
     n 1: a proposal intended to explain certain facts or observations
     2: a tentative theory about 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: {possibility}, {theory}]
     3: a message expressing an opinion based on incomplete evidence
        [syn: {guess}, {conjecture}, {supposition}, {surmise}, {surmisal},
         {speculation}]
     [also: {hypotheses} (pl)]