From Latin 'nebula' (mist, cloud), from PIE *nebhos — cognate with Greek 'nephos' and German 'Nebel' (fog).
A cloud of gas and dust in outer space, visible as a luminous patch or dark silhouette against other luminous matter; historically, any diffuse astronomical object.
From Latin 'nebula' (mist, fog, cloud, vapour), from Proto-Indo-European '*nébʰos' (cloud, mist). The word was used in medieval Latin for a cloudy spot on the cornea of the eye. The astronomical sense was established in the seventeenth century when telescopes revealed diffuse, cloud-like patches in the night sky that could not be resolved into individual stars. Before the twentieth century, 'nebula' was applied broadly
Until Edwin Hubble proved in 1924 that the Andromeda 'nebula' was actually a separate galaxy far outside the Milky Way, all diffuse celestial objects were called nebulae. The 'Great Debate' of 1920 between astronomers Harlow Shapley and Heber Curtis centred on whether these 'spiral nebulae' were nearby gas clouds or distant 'island universes.' Hubble settled the matter, and the word 'nebula' was narrowed