The Collaborative International Dictionary of English v.0.48:

Nebula \Neb"u*la\ (n[e^]b"[-u]*l[.a]), n.; pl. Nebulae
   (n[e^]b"[-u]*l[=e]). [L., mist, cloud; akin to Gr. nefe`lh,
   ne`fos, cloud, mist, G. nebel mist, OHG. nebul, D. nevel,
   Skr. nabhas cloud, mist. Cf. Nebule.]
   1. (Astron.) A faint, cloudlike, self-luminous mass of matter
      situated beyond the solar system among the stars. The term
      was originally applied to any diffuse luminous region.
      Now, technically, it is applied to interstellar clouds of
      dust and gases (diffuse nebula). However distant
      galaxies and very distant star clusters often appear like
      them in the telescope, such as the spiral nebula in
      Andromeda, known now to be a distant galaxy.
      [1913 Webster +PJC]

   2. (Med.)
      (a) A white spot or a slight opacity of the cornea.
      (b) A cloudy appearance in the urine. [Obs.]
          [1913 Webster]

WordNet (r) 3.0 (2006):

diffuse nebula
    n 1: a cluster of stars within an intricate cloud of gas and
         dust [syn: diffuse nebula, gaseous nebula]