nebula

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Astronomy A diffuse mass of interstellar dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation.
2. n. Astronomy See galaxy.
3. n. Pathology A cloudy spot on the cornea.
4. n. Pathology Cloudiness in the urine.
5. n. A liquid medication applied by spraying.

Century Dictionary and Cyclopedia

1. n. A luminous patch in the heavens, far beyond the limits of the solar system. Some nebulæ are resolvable into clusters, generally globular, in which the separate stars can be distinguished. These are for the most part in the Galaxy. The remaining nebulæ are of two types, according as their spectra are continuous or consist of bright lines. The latter class are greenish-blue, have fairly definite outlines, and show a tendency to concentration toward the galactic circle. Of the three brightest lines in their spectra two are unidentified, and one is the F line of hydrogen There are six or seven other faint lines, two of them hydrogen. There are besides nebulous stars, or stars with haze about them which in some cases is of vast proportions. The continuous spectra indicate that all these nebulæ are solid, liquid, or, if gaseous, enormously condensed. The nebulæ in Andromeda, Orion, and Argo are visible to the naked eye. The Galaxy, the Magellanic clouds, and the clusters Berenice's Hair and Præsepe are not included by astronomers among the nebulæ.
2. n. In pathology, a cloud-like spot on the cornea.

Wiktionary

1. n. A cloud in outer space consisting of gas or dust (e.g. a cloud formed after a star explodes).

GNU Webster's 1913

1. n. A faint, cloudlike, self-luminous mass of matter situated beyond the solar system among the stars. The term was originally applied to any diffuse luminous region. Now, technically, it is applied to interstellar clouds of dust and gases (diffuse nebula). However distant galaxies and very distant star clusters often appear like them in the telescope, such as the spiral nebula in Andromeda, known now to be a distant galaxy.
2. n. A white spot or a slight opacity of the cornea.
3. n. A cloudy appearance in the urine.

WordNet 3.0

1. n. cloudiness of the urine
2. n. (pathology) a faint cloudy spot on the cornea
3. n. a medicinal liquid preparation intended for use in an atomizer
4. n. an immense cloud of gas (mainly hydrogen) and dust in interstellar space