[1] : The Collaborative International Dictionary of English v.0.48

nebula \neb"ula\ ne^b"-ul.a, n.; pl. nebulae
   ne^b"-ul=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
see also:
nebulae nebule diffuse nebula spiral nebula 

[2] : WordNet (r) 2.0

spiral nebula
     n : a galaxy having a spiral structure; arms containing younger
         stars spiral out from old stars at the center syn: spiral
         galaxy
see also:
spiral galaxy 

Spiral >> S Words
Spiral, definition of term: Spiral
spiral_pag1.html

Nebula >> N Words
Nebula, definition of term: Nebula
nebula_pag1.html

Spiral Galaxy >> S Words
Spiral Galaxy, definition of term: Spiral Galaxy
spiral+galaxy_pag1.html

Crab Nebula >> C Words
Crab Nebula, definition of term: Crab Nebula
crab+nebula_pag1.html