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
    adj 1: of or relating to or resembling a nebula; "the nebular
           hypothesis of the origin of the solar system" [syn:
           nebular, nebulous]
    2: resembling a cloud [syn: cloudlike, nebular]