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

The Collaborative International Dictionary of English v.0.48:

Nebular \Neb"u*lar\, a.
   Of or pertaining to nebulae; of the nature of, or resembling,
   a nebula.
   [1913 Webster]

   Nebular hypothesis, an hypothesis to explain the process of
      formation of the stars and planets, presented in various
      forms by Kant, Herschel, Laplace, and others. As formed by
      Laplace, it supposed the matter of the solar system to
      have existed originally in the form of a vast, diffused,
      revolving nebula, which, gradually cooling and
      contracting, threw off, in obedience to mechanical and
      physical laws, succesive rings of matter, from which
      subsequently, by the same laws, were produced the several
      planets, satellites, and other bodies of the system. The
      phrase may indicate any hypothesis according to which the
      stars or the bodies of the solar system have been evolved
      from a widely diffused nebulous form of matter.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nebular hypothesis
    n 1: (cosmology) the theory that the solar system evolved from a
         hot gaseous nebula