1754, "cloudiness, haziness," from French nébulosité, from Late Latin nebulositatem (nominative nebulositas), from Latin nebulosus, from nebula "mist, vapor" (from PIE root *nebh- "cloud"). From 1761 as "faint, misty appearance surrounding certain stars."

mid-15c., nebule "a cloud, mist," from Latin nebula, plural nebulae, "mist, vapor, fog, smoke, exhalation," figuratively "darkness, obscurity," from PIE root *nebh- "cloud."

Re-borrowed from Latin 1660s in sense of "cataracts in the eye;" astronomical meaning "luminous cloud-like patch in the heavens" is from c. 1730. As early as Herschel (1802) astronomers realized that some nebulae were star clusters, but the certain distinction of relatively nearby cosmic gas clouds from distant galaxies (as these are now properly called) was not made until the 1920s, when the latter were resolved into individual stars (and nebulae) using the new 100-inch Mt. Wilson telescope.

Old English mistel "basil, mistletoe," from Proto-Germanic *mikhstilaz "mistletoe" (source also of Old Saxon mistil, Dutch mistel, Old High German mistil, German Mistel, Swedish mistel), a word of uncertain origin. According to Watkins, it is a diminutive form, so called because it "is propagated through the droppings of the missel thrush," from Germanic suffixed form *mih-stu-, "urine," hence "mist, fine rain," from PIE root *meigh- "to urinate." Missel-bird "missel thrush" is attested from 1620s.