Nebula \Neb"u*la\, n.; pl. Nebul[ae]. [L., mist, cloud; akin to Gr. ?, ?, 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. True nebul[ae] are gaseous; but very distant star clusters often appear like them in the telescope.

Source: Webster's Revised Unabridged Dictionary (1913)