nebulium: Meaning and Definition of

ne•bu•li•um

Pronunciation: (nu-by'lē-um), [key]
— n. Astron.
  1. a hypothetical element once thought to be present in emission nebulae because of certain unidentified spectral lines, now known to be forbidden transitions of oxygen and nitrogen ions.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.