New Latin, from Latin, mist, cloud; akin to Old High German nebul fog, Greek nephelē, nephos cloud
First Known Use: 1718
Medical Definition of NEBULA
: a slight cloudy opacity of the cornea
Any of various tenuous clouds of gas and dust in interstellar space. Nebulae constitute only a small percentage of a galaxy's mass. Dark nebulae (e.g., the Coalsack) are very dense, cold molecular clouds that appear as large, obscure, irregularly shaped areas in the sky. Bright nebulae (e.g., the Crab Nebula, planetary nebula) appear as faintly luminous, glowing surfaces; they emit their own light or reflect that of stars near them. The term nebula also formerly referred to galaxies outside the Milky Way Galaxy.