An Etymological Dictionary of Astronomy and Astrophysics

1. A cloud of gas and dust in the interstellar space. There are three general types: → emission nebulae,
   → reflection nebulae, and → dark nebulae.
   

2. A celestial body appearing nebulous or fuzzy when seen with the telescope. Formerly, galaxies, which appeared nebular but are constituted of billions of stars, were not distinguished from truly nebular objects, made of gas and dust.

Etymology (EN): From L. nebula “mist,” nimbus “rainstorm, rain cloud;” cognate with Av. napta- “moist,” nabās-câ- “cloud,” nabah- “sky;” Pers. nam “moisture;” cf. Gk. nephos “cloud, mass of clouds,” nephele “cloud;” Skt. nábhas- “moisture, cloud, mist;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark;” PIE base *nebh- “cloud, vapor, fog, moist, sky.”

Etymology (PE): Miq “nebula” (used by Tusi, in Pers. translation of Sufi’s “Book of Fixed Stars”),
variants meh “fog,” mož, Tabari miyâ, Lori/Laki (kara) mozy, Ossetic mig/megæ,
from Mid.Pers. mēq “cloud, mist;” Av. mēγa- “cloud;” cf. Skt. meghá- “cloud, overcast weather;” Gk. omikhle “mist;” Lith. miglà “mist, haze;” PIE base *mighlā- “cloud.”